NikonGear'23
Gear Talk => Lens Talk => Topic started by: Almass on July 13, 2016, 21:47:13
-
There are now two threads where there is mention that a lens of any focal length "magically morphs" into a sensor size factorial of 1.5 or whatever, thus giving additional reach.
Let's once and for all put this issue to re3st.
Focal length does not change on a full frame or cropped frame whatsoever.
What changes is the CrOP.
Let me explain for the visually impaired:
You have a 250gm steak on a plate surrounded with French Fries. You shoot the plate with a cropped sensor and you get only the steak, you shoot the plate with a full frame, you get the steak and the fries. The steak visual size does not change. (camera to subject distance being constant)
So if you have a 300P on a cropped sensor of 1.5 it does NOT make your lens a 300x1.5= a 450mm
and if you add a doubler of say factor 2, you DO NOT get a 900mm but a 600mm with fstop handicap factored in.
Just take you full frame camera and shoot any subject. Change your image size in the menu or change camera to cropped and shoot the same subject again keeping the same camera to subject distance. Now you have two pics. Crop the full frame to the same size of the cropped frame and.........magic both pics are the same size.....Dohhh.
If some would like to be anal about the issue. The only difference between a same lens used on FF or Dx, besides the crop, is in the DOF......which is very minute difference between the two but is still there for the discerning eye.
-
We all ought to know these basic facts and people still, to this day, muddle the issue. Perhaps they have a lens with ability to understand what camera it is attached to and thus can automagically change its optical inherent properties? Perhaps a magnifying element manifests itself from thin air and inserts itself into the optics? Must be magic or snake oil or both.
For old-timers having used anything from "small format" 24x36 mm up to 8x10", the belief in "crop factors" and change of focal length from one format to another is - well - unbelievable. The Internet is capable of making myths become an accepted "fact" if the errors are stated a sufficient number of times, apparently.
Oh well. Between the alternatives of bashing one's head against the wall in desperation or ignore the expressed nonsense, I prefer the latter one. Produces less headaches in the longer run.
Repeat 1000 times: focal length is focal length is focal length .... It is an inherent property of the optical design. It cannot change just by moving the lens onto another format. Period.
-
Thanks Bjørn. You made me definitely belonging to the "old farts" category i.e. I entirely agree with your analysis ...
-
Oh well. Between the alternatives of bashing one's head against the wall in desperation or ignore the expressed nonsense, I prefer the latter one. Produces less headaches in the longer run.
If I shoot birds and have to crop to one tenth using a 24x36-sensor of 21Megapixels with a 300mm lens I get a 2 Megapixel bird shot.
If I shot the same bird from the same position (photographers call it "perspective") using a 16x24-sensor of 21 Megapixels with a 300mm lens I get a 4.5 Megapixel bird shot.
If the 300mm lens is not up to the task, the details I get from the 16x24-sensor-4.5-Megapixels-crop are exactly the same as from the 24x36-sensor-2-Megapixel-crop.
If the 300mm-lens IS up to the task I get more details through more magnification like I get more details from a D810-File than I get from a D3-File.
Terminology can be confusing, but with some practice everybody shall be able to learn in the long run.
-
"If the 300mm-lens IS up to the task I get more details through more magnification like I get more details from a D810-File than I get from a D3-File."
Magnification and resolution are two entirely different concepts. Obviously people confuse them.
-
Also the myth of increased DOF (Depth Of Field) with DF sensors: if you want to frame the same scenery with a DF sensor (compared to a FX sensor) you have to maintain a bigger distance from the subject: increased distance = increased DOF. Or the other way around: with a bigger sensor and the same focal length, you get to be closer to the subject: shorter distance = shorter DOF. QED (Quod Erat Demostrandum).
-
Keeping the concept of an unaltered focal length firmly in mind suffices. The black voids of such bizarre ideas as 'equivalence' and 'equivalent' f-stops (sic) should be avoided as they only can throw mud in one's eyes. Some never recover.
-
Repeat 1000 times: focal length is focal length is focal length ...
Except when it isn't. Which in the case of any internal focusing lens, which is most modern lenses, is nearly all the time.
"Crop factor" is a metaphor. And none the worse for that - so are "light ray" and "depth of field" and "focal length". Problems arise in all those cases when people forget that those terms are metaphors and use them as the names of real things.
-
Except when it isn't. Which in the case of any internal focusing lens, which is most modern lenses, is nearly all the time.
"Crop factor" is a metaphor. And none the worse for that - so are "light ray" and "depth of field" and "focal length". Problems arise in all those cases when people forget that those terms are metaphors and use them as the names of real things.
Focal length is still focal length. The shortening when not at infinity is lens behaviour, not focal length ;).
-
Focal length is, like aperture (f-number), defined for infinity focus.
Many lenses these days are akin to zoom lenses in that their focal length alters when they focus from infinity to the near limit. Still whatever focal length the optics exhibit at a given focused distance will remain unchanged from one format to another. which after all is the main theme here.
-
Focal length is still focal length. The shortening when not at infinity is lens behaviour, not focal length ;).
Well, "lens behaviour" is a metaphor as well, and a notably vague one. So is "infinity", though not so vague.
The focal length of a lens is the distance from the rear principal plane to the plane in which the image of an object at infinity is in sharp focus. So focal length is a theoretical quantity because nothing is infinitely distant. Objects closer to the lens than infinity are in sharp focus at greater distances from the rear principal plane than the focal length, and an alternative way of putting it is that the focal length is the shortest distance from the real principal plane to the sensor/film at which anything can be in focus. That is a handy thing to know if you are using a view camera, but it has no particular importance for cameras with fixed lens to sensor/film distances. Focal length is used because everyone is used to it, but angle of view would at least as good and in some ways better as a descriptor (because then you would not have people thinking that an 18-140 lens has a bigger zoom range than a 16-80).
An internal focusing lens does not have a single focal length: it focuses closer by changing the focal length. Of course, for descriptive purposes we use the infinity focal length, but that is perfectly arbitrary (and handy for advertising, because that is always the longest focal length). But the focal length at infinity is no more the focal length than any other.
-
Will some people join who do not know the facts or will we knowers scratch each others back to infinity?
-
The German "brennweite" (burn length) and Dutch "brandpuntsafstand" (burn point distance) say it all. Much more descriptive than focal length... ;D
-
"But the focal length at infinity is no more the focal length than any other. "
Except for being the definition of focal length. Which in turn enters the definition of aperture (f-number).
One should probably give up at this point.
-
The German "brennweite" (burn length) and Dutch "brandpuntsafstand" (burn point distance) say it all. Much more descriptive than focal length... ;D
It's exactly the same concept and in fact the same nomenclature. Focal point = focused point = burn point, length = distance.
Thus 'focal length' == distance to burn point.
-
It's exactly the same concept and in fact the same nomenclature. Focal point = focused point = burn point, length = distance.
Thus 'focal length' == distance to burn point.
Of course.
But "Brennweite" is very easy to explain, also to kids, with only the sun ("at infinity"), a magnifying glass and some things that can burn/smoke. 8)
-
In the "burn point", light rays are focused, thus it is the focal point of the lens. Hence we are discussing exactly the same concept, just giving its designation in another language.
No need to complicate the matter.
-
It is more subtle than that. You have tried to make clear already a few times on this forum that focal length is a property that has only meaning at infinity.
Sunrays from infinity => lens => burn a hole in some thing at "brennweite", it automatically, inherently "includes" infinity.
I will not complicate things over here anymore.
-
No, it is the the rear conjugate distance 'v' that equals 'f' (focal length) when the front conjugate distance 'u' goes to infinity (the exact scenario you describe).
1/u+1/v=1/f (thin lens equation: internodal distance = 0)
Thus your assertion is misleading. Focal length 'f' has always "a meaning". It is a crucial component of the conjugate relationships of a lens.
-
Also the myth of increased DOF (Depth Of Field) with DF sensors: if you want to frame the same scenery with a DF sensor (compared to a FX sensor) you have to maintain a bigger distance from the subject: increased distance = increased DOF. Or the other way around: with a bigger sensor and the same focal length, you get to be closer to the subject: shorter distance = shorter DOF. QED (Quod Erat Demostrandum).
I haven't read all this thread yet, but just to address this one issue we can presume a crop is a crop is a crop. Of course, if you take an FX picture and a DX picture with the same lens at the same distance, and crop the FX to match the DX, the two should be the same. But if you're trying to duplicate images with the two formats without cropping, I think the DX will always have increased depth of field.
Sometimes I wonder if people are getting too tangled up in the technical details of optics and rays and theories, and forgetting that the thing we're trying to figure out is what image you'll get on the equipment you're getting it on. Optically, the myth of increased DOF may be just that, but from the practical point of view, if you want shallower depth of field for a specific image, you'll have a harder time getting it on DX.
Since both longer focal length and shorter distance increase depth of field, and of the two, focal length has greater effect, I can't imagine how a smaller sensor would ever result in shallower depth of field without a change in aperture. Between an FX and DX camera, the scene will be approximately duplicated with a 50 and a 35 (and yes, I know it's only approximately). The 50 will have shallower DOF at the same distance. If you take a DX camera and frame a subject the same with a 35 and a 50 (disregarding, of course, the different perspective), the 50 will still have a shallower depth of field even though it is further away.
-
We are back to the crux of the matter, namely, magnification of detail. When one changes the focal length but not the position, magnification changes and with it, depth of field. If you want to compensate by moving the camera and keep the lens, you alter perspective and thereby the perceived depth of field. Either way you cannot make these matters "equivalent" when formats differ and accordingly, the entire chain of arguments is shattered. It simply is an exercise in circular reasoning.
In the old days, photographers using different formats understood these principles and selected the format suited for their task. They also, hopefully, understood the concept of empty magnification, which is in the end what constrains the image quality any system can deliver if you push it outside its limits.
-
"But the focal length at infinity is no more the focal length than any other. "
Except for being the definition of focal length. Which in turn enters the definition of aperture (f-number).
.
Yes, but that is not the point.
The lens conjugate equation gives 1/f = 1/u + 1/v, where f is focal length, u is object distance and v is image distance, measured from the principal planes. If u is infinity, 1/f = 1/v, which is convenient, but focal length can be defined for any values of u and v. If u is not infinity, v has to get bigger if f is to stay the same, which is what happens with a view camera. But the equation shows also that it is possible to make u smaller without changing v by making f smaller: if f is smaller, 1/f is bigger, so 1/u +1/v is bigger, so v can stay the same although 1/u is bigger and u is smaller.
But how can you change focal length of a lens? Well, for any compound lens there is a simple lens of equivalent focal length. The equivalent focal length, f, of a compound lens made up of two lenses of focal length f1, f2 is given by 1/f = 1/f1 + 1/f2 if the two lenses are in contact. If the two lenses are separated by a distance d, 1/f = 1/f1 + 1/f2 - d/f1.f2. If you increase the distance between the lenses the equivalent focal length falls; that is how internal focusing works.
But the new equivalent focal length you get by changing the separation of the lenses is exactly the same "f" in the lens conjugate equation as the old equivalent focal length. It is - key point - the infinity focal length of a different lens.
-
I feel these points can be and already are beaten to their certain death. As we don't know the internodal distances for most lenses, any equation will be just an approximation anyway. This is very evident in the near range.
I recommend looking into the finder of the camera and take the shot. It answers all questions. Also the unchanged fact that the format of the camera does not in any way alter inherent properties of the lens ,which was if one recalls, the starting point for this thread.
-
Since both longer focal length and shorter distance increase depth of field, and of the two, focal length has greater effect, I can't imagine how a smaller sensor would ever result in shallower depth of field without a change in aperture. Between an FX and DX camera, the scene will be approximately duplicated with a 50 and a 35 (and yes, I know it's only approximately). The 50 will have shallower DOF at the same distance. If you take a DX camera and frame a subject the same with a 35 and a 50 (disregarding, of course, the different perspective), the 50 will still have a shallower depth of field even though it is further away.
DoF is inversely proportional to the square of focal length and directly proportional to the square of distance. The two have identical effects. DX will always have more DoF for the same framing because of the shorter focal length.
However, a smaller film negative has to be magnified more than a larger negative to give the same print size, so DoF is less in the print from the smaller negative. The same is true of digital sensors, assuming - as is usually ignored - equal output resolution. This is why the depth of field calculators use a smaller circle of confusion for DX sensors than FX.
The upshot is that for equal framing and equal print size and output resolution you need about 1 stop wider aperture with DX to get the same DoF as with FX. So if the DoF you want corresponds to an aperture at least one stop down from maximum, you can duplicate the FX DoF with DX (eg, 85mm on FX at f/2.8 is the same DoF as 58mm on DX at f/2). It is only if you want a DoF that requires an aperture near maximum that you can't get the same result in DX and FX.
But the bigger you print, the smaller the DoF, so if you can't use a bigger aperture to lower the DoF you can just print bigger (that ought to be cancelled out by people viewing larger prints from further away, which increases DoF, but in most circumstances people do not adjust their viewing distance to match print size).
-
It's quite easy to look at a scene with the naked eye, choose the distance to set perspective, more right or left, up or down and choose a point of view. A monorail view camera is more difficult to setup than a 35mm camera and I think that's where I learned this method. I don't always do this but I do it frequently.
This crop factor thing is like trying to speak a second language while translating each word into one's native tongue. This crop factor thing should pass soon but it's currently being reinforced almost everywhere.
Dave
-
"DX will always have more DoF for the same framing because of the shorter focal length. "
Why must a photographer use a shorter lens - is it mandatory for a DX camera? Of course not. Is the assumption of "the same framing" a firm non-negotiable constraint? Of course not, yet again. These are not prerequisites hewn in stone and hence the assertions and any conclusions thereof are void. To wit, I could double or triple the focal length with my DX camera and step back - then I would get *less* 'DOF'. And so ad nauseam.
The circular reasoning is riddled with unspoken assumptions and conditions that one can twist in any direction to gain support for whatever conclusion one wants. In the longer run, this gets very boring.
-
To me owning camera's with different sensor sizes and resolutions and different lenses - as most here on the forum I think - it is quite logical to compare them in terms of field of view, DOF, magnification and resolution for different applications like birding, macro, portraits and landscapes amongst others.
Sure every comparison has shortcomings but it helps me to pick the right camera/lens combinations for an outing. And I think the following well thought through article by dpreview at least helps in some of these considerations. http://www.dpreview.com/articles/2666934640/what-is-equivalence-and-why-should-i-care (http://www.dpreview.com/articles/2666934640/what-is-equivalence-and-why-should-i-care)
But I am sure several of you will find significant flaws in it :-[
-
Some stuff is better left untouched. Too many worms in it.
By all means, read and enjoy 'net discussion pro et contra at your heart's desire. As long as you get the pictures you wish, nobody should bother about your choices or the fallacious claims put forward in these debates.
-
DoF is inversely proportional to the square of focal length and directly proportional to the square of distance. The two have identical effects. DX will always have more DoF for the same framing because of the shorter focal length.
However, a smaller film negative has to be magnified more than a larger negative to give the same print size, so DoF is less in the print from the smaller negative. The same is true of digital sensors, assuming - as is usually ignored - equal output resolution. This is why the depth of field calculators use a smaller circle of confusion for DX sensors than FX.
The upshot is that for equal framing and equal print size and output resolution you need about 1 stop wider aperture with DX to get the same DoF as with FX. So if the DoF you want corresponds to an aperture at least one stop down from maximum, you can duplicate the FX DoF with DX (eg, 85mm on FX at f/2.8 is the same DoF as 58mm on DX at f/2). It is only if you want a DoF that requires an aperture near maximum that you can't get the same result in DX and FX.
But the bigger you print, the smaller the DoF, so if you can't use a bigger aperture to lower the DoF you can just print bigger (that ought to be cancelled out by people viewing larger prints from further away, which increases DoF, but in most circumstances people do not adjust their viewing distance to match print size).
Maybe I'm getting stuck on terminology or theory or something, but if I take a DX camera, and put a 50 mm.lens on it, and take a picture of a subject, and then put a 35 mm. lens on it at the same aperture, and move the 35 mm. lens closer to the subject to duplicate the size of that subject in the given sensor size, I will get two results: one is that of different perspective, the wider lens showing a much wider background, and the second is that the view with the wider lens will have greater depth of field.
In other words, as far as I can see in the practical sense of what you get when you make a picture, the effects of focal length and distance do not seem to cancel out.
-
It is possible to get equivalent pictures with FX and DX cameras:
- the two cameras must be shot from the same point (same perspective)
- the field of view of the lenses must be identical, for example 35mm on DX and 50mm on FX
- the entrance pupil of the two lenses must be identical, for example shoot the 35mm at f/2.8 and 50mm at f/4 (35/2.8 = 50/4 = 12.5mm)
In other words when comparing DX and FX, the DX lens must have a focal length one "stop" shorter, and the aperture must be one stop faster to compensate. This is only an approximation, one stop is sqrt(2) or about 1.4, while the difference between DX and FX is about 1.5x linear, but it's close enough for most practical purposes.
This will give two images with the same framing and same relationship between the focus plane and out of focus foreground/background (DOF, bokeh etc) This is because the two cameras are viewing the world through the same "window" - the same entrance pupil from the same position. The only difference is how the view from the window is projected onto the sensor - in one case it is projected further away over a larger area (longer focal length on bigger FX sensor), in the other case projected closer over a smaller area (shorter focal length on a DX sensor). Notice also that when the image is projected over a smaller area, the light intensity is higher, in other words the f/stop is faster.
This assumes the two lenses have similar rendering, depending on how the lens is corrected the DOF and bokeh may differ greatly even for otherwise similar focal lengths and apertures (see the thread comparing the ADS 85/1.8 with the Tamron 85/1.8). This comparison also ignores differences at pixel level. If both sensors have the same pixel pitch the smaller sensor will obviously have fewer pixels, it's basically a crop of the larger sensor, which puts greater limits on how much the image can be enlarged. If both sensors have the same number of pixels, the smaller sensor obviously has smaller pixels, which means each pixel has less light-gathering capacity, may have less dynamic range etc
-
Notice also that when the image is projected over a smaller area, the light intensity is higher, in other words the f/stop is faster.
I'm half asleep but that sounds fishy. I can hear my Pentax Digital Spotmeter grumbling in the cabinet. Well I'm off to bed. :)
Dave
-
Light intensity is the same, the total amount of light captured is lower on DX vs FX.
-
"DX will always have more DoF for the same framing because of the shorter focal length. "
Why must a photographer use a shorter lens - is it mandatory for a DX camera? Of course not. Is the assumption of "the same framing" a firm non-negotiable constraint? Of course not, yet again.
Of course not indeed; it is very difficult to think of any plausible circumstances where someone would have a reason to take the same picture with a DX and an FX camera. And it always was: the whole DoF and format canard started as a reason to buy Canon instead of Nikon when Canon had 36 x 24 sensors and Nikon did not. But, sad as it is, DoF-and-format now has a life of its own and - perhaps due to its origins as advertising - is poorly understood.
-
It is slightly ironic that amongst photographers there is no (or very little) consensus on matters of physical optics that have not been up for debate for two centuries.
I think this is more a case of human psychology than of the technicalities of the debate. Photographers have certain questions regarding the behaviour of their gear. They pick the argument that answers their questions in the simplest and fastest way, not the argument that is soundest in terms of stating every implicit assumption explicitly. Neither they pick the argument that emerges from historical photographic practice. Hopefully they verify for themselves that the predictions of the 'theory' they picked is actually correct (which is not all too easy with all the confirmation bias coming in). If that is the case, all is well. If not, they are probably on a fool's errand or it doesn't actually matter all that much.
As for the format comparisons and such: format comparisons do serve a different purpose than they did with view cameras 50 or more years ago (I surmise, because I was not there). Now many people compare formats expecting to use them for the near-same purpose exclusively because sensors are not modular (unlike film backs) and you are stuck with what you have in your camera body. Say, you compare putting your money in a FX vs. a m4/3 system and only one of them. Of course they are very different, but if you have to choose, what do you do? All sorts of questions pop up regarding the relative benefits and drawbacks. Among these are optical limitations of the lenses and how they translate into visible differences in the final pictures (that are viewed without knowing the gear that was used to create them, hence expectations and output sizes are often gear-invariant whereas the obtainable results are not). In these cases, excessive relativism and harping on obvious things like "each format has its purpose and use" or "if you know your thing, you can make any camera sing" does not answer these concerns, you still have to balance everything and come up with one answer.
-
I'm half asleep but that sounds fishy. I can hear my Pentax Digital Spotmeter grumbling in the cabinet. Well I'm off to bed. :)
Dave
If people were to try using "equivalent" apertures with an external light meter, hillarity ensues for sure. :D
-
Light intensity is the same, the total amount of light captured is lower on DX vs FX.
The "total" doesn't matter - areas to be illuminated are different. Photography deals with intensity of light. Not the "total".
Why do we have the f-number scale as a relative figure such as f/4? The absolute sizes of the aperture inside the lens are vastly different on a 600mm f/4 and a 60mm f/4 lens ... so the longer lens "lets in" much more light overall. Does that imply we need a different exposure when we switch from one to another of them? Not at all. The longer lens collects precisely the same intensity of light as the shorter lens and delivers the same intensity at the film plane as well. That is why a light meter does not need to be separate for each focal length *and* format.
These facts have been known to, and used by, photographers for centuries. Why human kind in the contemporary times when virtually everyone has a camera or picture-taking device starts disbelieving the fundamentals of photography to replace it with snake oil arguments is beyond me. Is there no sense of history and accumulated insights? I hesitate to blame the internet entirely for this sad state of the affairs.
-
Actually total light captured by a sensor matters a lot. Not in terms of settings of the camera of course, there light intensity is the driver.
But as FX camera's capture more light they usually perform better at low light than DX camera's and you cannot blaim the internet for this.
-
to all: just wait a second - i need to buy popcorn
-
It's quite easy to look at a scene with the naked eye, choose the distance to set perspective, more right or left, up or down and choose a point of view. A monorail view camera is more difficult to setup than a 35mm camera and I think that's where I learned this method. I don't always do this but I do it frequently.
I do it all the time. Even with my mobile phone. I feel the results get better.
-
Maybe I'm getting stuck on terminology or theory or something, but if I take a DX camera, and put a 50 mm.lens on it, and take a picture of a subject, and then put a 35 mm. lens on it at the same aperture, and move the 35 mm. lens closer to the subject to duplicate the size of that subject in the given sensor size, I will get two results: one is that of different perspective, the wider lens showing a much wider background, and the second is that the view with the wider lens will have greater depth of field. In other words, as far as I can see in the practical sense of what you get when you make a picture, the effects of focal length and distance do not seem to cancel out.
You are totally right. I had to shoot a grop in front of the church. took it with the D500 & 1.4/24G and FROM THE SAME PERSPECTIVE with the D600 and 1.4/35 Ai-S ... the results are very different BECAUSE the lenses draw very differently (LENS CHARACTERISTICS) and the BG is rendered differently in the compression department (PHYSICS)
-
If anyone wants a sensible discussion of "equivalence", then I recommend the following: http://www.josephjamesphotography.com/equivalence/index.htm (http://www.josephjamesphotography.com/equivalence/index.htm). Within Mr. James essay there is a nice summary (see Introduction) and also in-depth discussions. I've enjoyed his approach to the topic. There is a lot there, and it takes some time to read through.
While there is a lot of snake oil floating around, we cannot totally ignore the DX vs FX debate until we each understand it fully.
-
I'm half asleep but that sounds fishy. I can hear my Pentax Digital Spotmeter grumbling in the cabinet. Well I'm off to bed. :)
Read Roland's post once more when awake, and you will hopefully understand that your Spotmeter will agree with Roland.
-
But as FX cameras capture more light they usually perform better at low light than DX cameras and you cannot blaim the internet for this.
This needs an explanation.....because you can set up an FX camera and a DX camera in such a way that for a given subject they both receive exactly the same amounts of total light even if the light, generally speaking, is "low". In such a setup, why would the FX perform better than the DX? ;D
(I am making the assumption of equal light gathering efficiency in both the FX and DX sensors.)
[Made an edit to remove statement about light density. I was getting carried away.]
-
Andrea, I meant at equal exposure a FX sensor captures more light than a DX sensor and I quote from your link to the interesting essay of Mr. James (have not read it all yet, that will take some time and energy..):
The reason that smaller sensors are more noisy than larger sensors is not because they are less efficient, but because less light falls on them for a given exposure. If the larger sensor is more efficient than the smaller sensor, then the noise gap will widen, if the smaller sensor is more efficient, the noise gap will shrink.
-
To do that you need a 1.1 stops faster lens on the DX camera, which may or may not exist let alone perform well at that aperture. Typically it is easier to make a higher performing slower lens for a larger format than a faster, shorter focal length lens for a smaller format, in a way that wide open performance is sufficient.
-
Exceptions for DX are the new Sigma Art zooms (18-35/1.8 and 50-100/1,8) that perform as well wide open at f/1.8 as their FX counterparts at f/2.8 (if only there were no focus issues with these Sigma's).
-
Arend ;D .
Didn't Sigma offer a firmware fix? I thought I read that somewhere.
Yes, the James article is a good write-up I think.
-
Photography deals with intensity of light. Not the "total".....
These facts have been known to, and used by, photographers for centuries. Why human kind in the contemporary times when virtually everyone has a camera or picture-taking device starts disbelieving the fundamentals of photography to replace it with snake oil arguments is beyond me. Is there no sense of history and accumulated insights? I hesitate to blame the internet entirely for this sad state of the affairs.
hear, hear! The aegis of the internet can only exacerbate the problem : the maxim of the late, unlamented Herr Goebbels > tell a lie often enough, and it becomes widely believed.
Yet one should not be surprised, it is hard to find a discipline within science itself - life sciences especially - that does not suffer failures to learn from historical lessons, and even the core concepts of the discipline. What passes for the education system, even in postgrad, is at fault of course; it seems less and less workers within the field have read (or even have heard) of what is judged 'old stuff'. Trendy topics in vogue, and soundbites rule. Equally, to quote the late Peter Medawar 'Lucky Jim', his review of the Double Helix: "The history of science bores most scientists stiff...". This seems to hold for what is judged today as out of fashion, as much as the pioneers, let alone what knowledge they bequeathed to humankind.
And it seems even those who studied optics in high school physics last thought about the subject decades back; it's easier not having to consider core concepts.
-
Yes, required reading, where too many of us have used focal length as the wrong proxy to try and grope toward equivalence
If anyone wants a sensible discussion of "equivalence", then I recommend the following: http://www.josephjamesphotography.com/equivalence/index.htm (http://www.josephjamesphotography.com/equivalence/index.htm). Within Mr. James essay there is a nice summary (see Introduction) and also in-depth discussions. I've enjoyed his approach to the topic. There is a lot there, and it takes some time to read through.
While there is a lot of snake oil floating around, we cannot totally ignore the DX vs FX debate until we each understand it fully.
-
Notice also that when the image is projected over a smaller area, the light intensity is higher, in other words the f/stop is faster.
I believe the thinking here is you are using a larger aperture to get a similar image, background blurring, DoF, etc.
For example if I use a 70/2.8 lens on DX at the same f/2.8 I'll have less background blurring than I used a 105/2.5 at f/2.8 on DX. I knew what to expect when I bought my first dSLR which was a DX, all Nikon made at the time but still it was a let down. I had to use my 85/2.0 to get the background blurring I wanted in certain situations but now my perspective was flatter as I had to background to get the same crop.
Light intensity is the same, the total amount of light captured is lower on DX vs FX.
A larger format captures more light total at a given shutter speed and f/stop as it has more surface area to capture it on. That's what I'm reading here.
The total light captured affects the noise which when increased affects the dynamic range and resolution of the capture as noise replaces useful data.
Dave
OK, now I'll drink my first cup of coffee and start reading. :)
-
Dynamic Range v. ISO for my D300s and D800 in both DX and FX modes. (http://www.photonstophotos.net/Charts/PDR.htm#Nikon%20D300s,Nikon%20D800,Nikon%20D800(DX))
Based on DXO's damned lies these are my "three" dSLR(s). Notice that the D800 in DX mode has lower dynamic range for a given exposure (ISO, shutter speed and aperture) than my D800 in FX mode.
Dave
"There are three kinds of lies: lies, damned lies, and statistics." --Benjamin Disraeli
-
@Andrea, Sigma offers some fixes by sending the lens in (so not via the dock) but those who did report some improvement but still no reliable autofocus at all focuspoints (so I exchanged my Sigma 18-35 with a Nikkor 20/1,8).
@David, I guess the D800 DX and FX mode DXO numbers support the argument of a larger sensor capturing more light at the same exposure and thus offering higher quality.
I don't think I will have the energy to read the entire essay of Joseph James on equivalency, but I think he has some conclusions we can all agree to:
Photography is all about the photo.
The bottom line is that we use a camera to create photos. It is important to understand the advantages of any particular system as a whole, both in terms of IQ and operation. The purpose of equivalence is to help evaluate the IQ end of that consideration, and, in conjunction with our individual "quality threshold", make an informed choice as to which system, or systems, best meet our personal needs for the photography that we do.
-
FX capture "more light". Yes, of course, they must do because have a larger area to cover than their DX cousins. Intensity at the film plane is precisely the same. First and most fundamental law of Photography.
Can we debunk this myth and move on, please. Any difference between FX and DX is not found here.
-
this is very easy:
If you print 100cm x 70cm from a file containing information of 30 Billion Photons it looks much better than if you print 100cm x 70cm from a file containing information of 3 Billion Photons.
Format? NO
Pixel Budget? YES
Calling AndyE for help....
-
Now you add resolution into the pot for more stir'n'fry dishes ...
Not tempted.
Besides, there is an alternate interpretation of your assertions that expose a huge loophole in the reasoning. I wonder when you discover it yourself?
-
FX capture "more light". Yes, of course, they must do because have a larger area to cover than their DX cousins. Intensity at the film plane is precisely the same. First and most fundamental law of Photography.
Can we debunk this myth and move on, please. Any difference between FX and DX is not found here.
Thank you.
-
Thank you.
You are welcome. We aim to keep a lively debate thus as long as the NG Guide Lines are followed, any opinion can be expressed. However, at the same time some discussions become futile by entering into a state of endless repetitions. At that stage, it might be more prudent to cut off further activity.
-
Notice also that when the image is projected over a smaller area, the light intensity is higher, in other words the f/stop is faster
I'm half asleep but that sounds fishy. I can hear my Pentax Digital Spotmeter grumbling in the cabinet. Well I'm off to bed. :)
I wasn't very clear. If the "window" on the scene (entrance pupil) is the same , then obviously you gather the same amount of light. It's at the sensor end that you see the difference. If the same light is projected onto a small sensor, the light intensity must be more concentrated - greater brightness per unit area, in other words a faster aperture. When projected into a larger sensor (angle of view remains the same), the light is more spread out - lower brightness per unit area, slower aperture. That's why a DX lens must be about a stop faster than an equivalent FX lens to reproduce the same scene (when the two images are printed to the same size, both will have the same apparent DOF/Bokeh, assuming the lenses are corrected similarly). For example a DX 35/1.8 at f/2.8 will produce more or less identical image as an FX 50mm lens at f/4.
-
The lens does not act as a light energy amplifier, only an attenuator. It attenuates because it cannot collect more light that what is present at the entrance pupil. Thus the notion of intensity becoming greater over the entire image plane is more than questionable, it is untenable. (Light amplifier devices as in night vision are different as they use electronics and trade more intensity for restricted bandwidth and more noise. Thus they raise the gain of the entire optical system. A lens is a passive device, however).
A true point light source can be focused into an equivalent focal point ("burn point" for the sun, which is at infinity focus). At that single point in the film plane, intensity of the imaged scene (not the total light of the source) is increased. However, an image is created by many points. One cannot have the light intensity of all of them being greater at the same time over the time average of a photographic exposure. That would imply the lens creates light on its own.
An illustration (from underwater optics*, in which the water surface is the refractive interface): we are all familiar with the reticulated pattern of lights spots travelling over the bottom of the shallows or the swimming pool. These light trains are focused sun light. When one measures the spatial distribution of these underwater "light flashes", they do focus at a depth given by the curvature of the water surface in a very small area (actually, the surface film has a topography of fractal nature). Around their focal points, the light intensity can surpass that at the surface for a brief period of time. Their time averaged intensity, however, is precisely the attenuated fraction of incident irradiance commensurate with the optical parameters of the water body. In fact, the reticulated pattern is caused by most of the incoming light being defocused and hence reduced in intensity.
* studies conducted at the NIWA research facilities at Christchurch, NZ, while I was a visiting scientist there in the mid '90s
-
Where did anyone say that a lens was a light energy amplifier??
*****
Anyway, what Roland said is exactly how I understand it.
Shooting an FX cam and a DX cam at the same distance from the subject you can gather equal total amounts of light over both the FX and the DX sensors by ensuring you have the same framing and the same entrance pupil (aperture diameter) and the same shutter speed on the two systems.
The FX system will require a lower exposure to accomplish this. And the density of the total light will be less over the larger FX sensor than it is over the smaller DX sensor (higher exposure).
From Roland's example: entrance pupil = 35/2.8 = 50/4 = focal_length/f_ratio
If you set equal exposures instead of equal entrance pupils, then you get an equal density of light over each sensor but different amounts of total light (with the FX sensor seeing more total light, of course).
The point being that we can't generalize and say that "FX sensors gather more light than DX sensors". You have to qualify that with the correct references to distance, framing, entrance pupil and exposure settings.
-
The D600 counts 3 times as many photons per picture than the D3 in the same situation.
She has twice as many pixels on the same area too.
The "pixel budget" a value introduced by AndyE many years ago leads to finer tonality and/or finer spatial
resolution. The electronics converting photon counts to colored pixels has better input measurements to
start with. Each photosite is a measuring instrument whose results are inevitably poisson distributed meaning
the uncertainty of the result .. margin of error .. goes with roughly the square root of the measured result.
The bigger the absolute photon count of a photosite the smaller the uncertainty.
Now the designer of the recording chip can decide to either have a finer tonality like in the D750 or at the same
pixel budget a finer spatial resolution like in the D810.
That is where pixel buget ... better statistics ... finer tonality ... OR ... finer spatial resolution ... touches resolution.
In other words. If the instrument can distinguish 1 Million shades of red or only 50.000 .... OR ... if the instrument can in
stead make even the finest of hairs countable.
-
Well, what you now describe has absolutely nothing to do with the starting point of this thread.
Sensor quality has nothing to do with focal length. And focal length on its own does not depend on format.
The circle of confusion is now well and truly completed.
-
The importance of a larger format gathering more total light is signal to noise ratio.
More damned lies...
D810 FX v. D810 DX vD810A FX (http://www.photonstophotos.net/Charts/PDR.htm#Nikon%20D810,Nikon%20D810(DX),Nikon%20D810A)
And again...
D5 FX v. D5 DX v. D500 (http://www.photonstophotos.net/Charts/PDR.htm#Nikon%20D5,Nikon%20D5(DX),Nikon%20D500)
More signal: more dynamic range, more image resolution, more image acutance.
More noise: less dynamic range, less image resolution, less image acutance.
If you want DX to equal FX in dynamic range you're going to need a bigger boat your going to need a faster lens.
Dave
-
Sensor quality has nothing to do with focal length. And focal length on its own does not depend on format.
The circle of confusion is now well and truly completed.
Finally, at least something got completed in this thread ;)
-
Why must a photographer use a shorter lens - is it mandatory for a DX camera?
If I want the same subject framing and the same perspective then I'll use a 70mm lens on DX and a 105mm lens on FX and I'll lose background blurring on DX as my fastest 70mm lens is an f/2.8 and my fastest 105mm lens is an f/2.5. If I'm willing to have flatter perspective then I can use an 85/2.0 lens from a greater distance to gain the same subject framing and I'll gain background blurring but I'll have flatter perspective which I won't want if the subject is a person. I've loved the 105mm focal length since the day I tried a 105/2.5 Nikkor-P in the mid '70s.
Twice I started a Nikon system with a 55/3.5 Micro followed by a 105/2.5 and then a 24/2.8. I bailed out of my first Nikon system just before Nikon started advertising multi-coated lenses.
Although I knew what to expect I was still disappointed with 70/2.8 on DX in term of background blurring as compared to a 105/2.5 or 105/2.8 lens on 35mm.
My 2 cents on why use shorter lenses on DX.
Dave
I wish the "crop factor" thing would go way but I don't think it will.
-
The D600 counts 3 times as many photons per picture than the D3 in the same situation.
But to take advantage of that you need to use a longer exposure. The base ISO of the D610 is 100 and the base ISO of the D3 is 200. If you use the same exposure and ISO then the difference is smaller.
-
Exceptions for DX are the new Sigma Art zooms (18-35/1.8 and 50-100/1,8) that perform as well wide open at f/1.8 as their FX counterparts at f/2.8 (if only there were no focus issues with these Sigma's).
. I respectfully disagree. While I do not know 18-35 lens, I do know 50-100 one very well. Wide open, especially to shorter distances, it can't create sharp image, above the central ring, to extreme corners, due to huge field curvature and associated geometrical distortions. It is unavoidable for such bright zoom. I am not able to say it is bad. For some applications, portraiture, for exmp., it is more than acceptable. Even closing down can't significantly improve that situation, you need to focusing further. 50-150 new Sigma is way better(different?) on this respect, as well as every good 70-200/2.8 lens for FF field. LZ
-
@ longzoom, actually I only have practical experience with the 18-35 and not the 50-100.
With the 18-35 I was actually very happy with its close-up performance wide open, also in the corners. And it may have had some field curvature but for the way I used it only needing part of my subject in focus I was very happy with its sharpness and rendering. And it was actually slightly better wide open than the Nikkor 20/1.8G that I now use, but not having reliable autofocus in the corners was not acceptable for me.
-
I think I will stop reading this topic, because the more I read, the more confused it becomes in my mind 😜
-
It's all about circles of confusion anyway ....
Will more photons improve these circles ??
-
I think I will stop reading this topic, because the more I read, the more confused it becomes in my mind 😜
MF, this is such an emotive issue on almost all Sites. I often shoot both formats in tandem. When I view the images they can go either way as to which turn out better (lots of contributing factors but most often the difference is me and how I'm feeling/shooting ).
I just do not normally even open the threads. The title of this one caught me out. Logic, science be damned ... lol
-
When one discusses this topic, it should be clear what the goal is. Otherwise we will run in circles.
If the goal is to reproduce the same photograph (same perspective, same field of view, same DOF, same motion blur (implies same shutter speed) with two formats (using the full frame without cropping) and display them both at the same size, then the exposures must be different for the two formats. The photographs will have the same brightness and noise level if the ISO is chosen appropriately for each format (different ISO for different formats).*
- What does it imply for the importance we should place on exposure in this context?
- Does anyone disagree that the two photographs look the same under any circumstance or with any viewing device?
Does anyone disagree that we can deduce from the above that if the exposures are the same for the two formats, we either get
- different DOF or
- different motion blur,
but we definitely get
- different noise levels,
- the same perspective.
(it is assumed that ISO is the same here such that image brightness is the same).
The noise amplitude is lower for the bigger format and the information theoretic reason for that is that more photons were collected by the bigger sensor if both sensors were exposed the same (same energy per unit area).
* All these considerations are made under the simplifying assumption that sensor technology is similarly mature for both sensors and that things like the exact architecture or number of the photosites are similar enough to be neglected. We also assume that we can find lenses for each format that project geometrically similar images and are diffraction-limited. All statements are approximate to the extent that these assumptions are violated, but they shouldn't usually be violated by orders of magnitude.
-
Back when I didn't know the role different sized sensors played in making photographs I saw a diagram similar to the amazing diagram below which I've (re)created just for this very discussion.
This sums it up good enough for me and I can't imagine how any more knowledge on the subject would improve my photography in real world shooting.
If you're trying to write a text book that's one thing, but if you're just making photographs it's really not that complicated. Pick up any camera, point it at something interesting.
-
The sheer insistence on "the same" is about as useful and productive as demanding every photographer being at a given location should return with the exact same photograph.
Isn't time to do something useful now?
-
As far as I can see, this topic is not about photography specifically. It is about optics and digital sensors. Photography just so happens to make use of these tools to communicate a message. There are great artists who do everything intuitively and don't care about the science behind it. But that's beside the point.
There is no artistic aspect to this topic as far as I can see. It is purely technical/scientific.
It is only because I see that we are lacking a consensus on the technical matters that I am bothering to write anything at all.
If what I write is not helpful or does not stimulate any thoughts/discussions, it is quite easy for me to quit doing it. As you suggest, I have plenty of other useful stuff to do with my time.
-
My reply wasn't directed towards you. I spoke on a general note. I also note that people rarely if ever speak of the reasons why this topic should be so important.
A horse can be beaten to death repeatedly, but the efforts are pretty much wasted after the first round.
-
Ok, I do not take it personally but I felt addressed because I had used the word 'same' a couple of times.
My view is that the topic of format comparisons is important is because many people do compare formats for all kinds of reasons, and some of them strive to do it in a rational manner. Often, the comparison is done in an either/or way because the person doing it can afford to put money in one and only one system, and hence has to choose a format over every other. The person perhaps wants to know the potential implications of this choice. They want to know whether system A allows them to do the same or similar as system B, or whether there are limitations to system A that do not affect system B. I do not judge anyone for having these thoughts -- I simply observe that they occur. I think the best we can do is to provide a framework to aid in these decisions, rather than to assert that the questions are invalid/unimportant.
Even to those who only ever use one camera system with one sensor size, the topic is important to understand the implications of cropping.
People who understand the basics can use them to choose the right lenses within their system.
I am sure other people could list more motivations here.
Anyway, the assertions that we make regarding relationships between different formats (e.g. the one in my post above or the ones in the treatise of Joseph James) are either true or false regardless of the importance of their truth or falsehood for individual photographers (artistically speaking). Thus we should be able to reach a consensus within this educated community about what is true and what is false. I think we can only profit if everyone participating in the discussion explains in detail why he/she thinks that a given assertion is false.
-
As far as I can see, this topic is not about photography specifically. It is about optics and digital sensors. Photography just so happens to make use of these tools to communicate a message. There are great artists who do everything intuitively and don't care about the science behind it. But that's beside the point.
There is no artistic aspect to this topic as far as I can see. It is purely technical/scientific.
It is only because I see that we are lacking a consensus on the technical matters that I am bothering to write anything at all.
If what I write is not helpful or does not stimulate any thoughts/discussions, it is quite easy for me to quit doing it. As you suggest, I have plenty of other useful stuff to do with my time.
Please continue. I find your comments helpful. As there is no definitive statement on this thread, I remain somewhat confused.
I think it is important to understand the effect of different sensors on the final image.
To give a real world example. When I go out on the streets, I like the perspective and framing of a 50mm lens on FX. If I wish to carry a DX camera, to keep roughly the same perspective and framing then I think I need to use a 35mm lens. I cannot use a 50mm and step back to preserve the framing, because perspective will change, and also there is typically no room to step back sufficiently. I think that, assuming an evenly illuminated scene, the aperture and shutter speed should be the same for both formats. But if I want the same DoF with DX as with my FX equipment, I will have to increase the aperture by one stop, and therefore have to increase the shutter speed by one stop. I also think that there will be more noise with the DX image, which will become more noticeable in low light conditions. For this purpose I assume same generation up to date Nikon technology.
Is my analysis correct?
-
Simsurace: That assumes a person consider the question worthy of attention in the first place .... Personally, I couldn't care less. One does what the camera does best, or if one only has a given gear at disposal, makes the most out of the situation. Period.
These "theoretical" considerations really boil down to idle speculation and in most cases, circular reasoning.
I have done my photography with camera formats from 1/3" to 8x10". The current frenzy of equivalence never entered the playing field, not once.
-
Please continue. I find your comments helpful. As there is no definitive statement on this thread, I remain somewhat confused.
I think it is important to understand the effect of different sensors on the final image.
To give a real world example. When I go out on the streets, I like the perspective and framing of a 50mm lens on FX. If I wish to carry a DX camera, to keep roughly the same perspective and framing then I think I need to use a 35mm lens. I cannot use a 50mm and step back to preserve the framing, because perspective will change, and also there is typically no room to step back sufficiently. I think that, assuming an evenly illuminated scene, the aperture and shutter speed should be the same for both formats. But if I want the same DoF with DX as with my FX equipment, I will have to increase the aperture by one stop, and therefore have to increase the shutter speed by one stop. I also think that there will be more noise with the DX image, which will become more noticeable in low light conditions. For this purpose I assume same generation up to date Nikon technology.
Is my analysis correct?
Your analysis is correct in my opinion, and one could add the following two points:
- You changed your aperture to match DOF but you also changed your shutter speed, which is ok for a static scene but might have unwanted consequences for moving subjects. To get the same amount of motion blur you are not allowed to change your shutter speed. You therefore get double the exposure than you wanted, but unless you are at base ISO, you can use that extra stop to lower your ISO by one stop. You will then match the noise of the FX cam (assuming the same technology). Thus, you did your best to make your DX photograph look as similar as possible as the FX photograph, and this is what you wanted. Great!
- If the scene does not move or indeed you like the shorter shutter speed to freeze the movement, you can do as you suggested, and have the same exposure for DX and FX. As you say, the noise is expected to be higher for DX than for FX. Inquiring minds want to know why...*
* There are various (equivalent) ways to explain this, one of them being that the bigger sensor registered more photon events, hence has more information to process than the smaller sensor.
-
Simsurace: That assumes a person consider the question worthy of attention in the first place .... Personally, I couldn't care less. One does what the camera does best, or if one only has a given gear at disposal, makes the most out of the situation. Period.
These "theoretical" considerations really boil down to idle speculation and in most cases, circular reasoning.
I have done my photography with camera formats from 1/3" to 8x10". The current frenzy of equivalence never entered the playing field, not once.
Still, the same physics must have affected the outcome of your photographs that affects all of our photographs, whether we are thinking about it or not. I do not see the conflict :)
-
There is no conflict. I'm aware of what I'm doing.
-
The noise amplitude is lower for the bigger format and the information theoretic reason for that is that more photons were collected by the bigger sensor if both sensors were exposed the same (same energy per unit area).
Actually the noise itself is higher for the larger format. The noise varies as the square root of number of photons which means that the signal-to-noise ratio (SNR) varies in the same way. So the larger format has a higher SNR and it is this which manifests itself in a "less noisy" picture.
I mention this not in the cause of pedantry but because Eric Fossum made the same point to Great Bustard in a DPRev thread ( http://www.dpreview.com/forums/post/57949779 ) recently. Great Bustard is the user name of Joseph James, one of the promoters of "Equivalence". Eric Fossum is the designer of some of the CMOS architecture which is found in most of our sensors. So, if I'm viewed as a snake oil salesman, I find myself in good company.
I am of course in agreement with your ideas and and have argued for them previously on the earlier Nikon Gear site - now Fotozones. I have found the ideas expressed by James and others to be very helpful in correcting many misunderstandings which appear to have infilltrated the field of photography. I could give a detailed account but I fear that there is little interest here.
-
Yes, thanks!
SNR is the proper term to talk about.
-
There is no conflict. I'm aware of what I'm doing.
There is no question that you are. But I'm wondering then why you disagree so strongly with some basic assertions (like e.g. that the absolute number of photons collected is a relevant parameter in some arguments, vs. just exposure). Since I believe that you understand the argument, it must be that it strongly contradicts your observations. Even if you don't normally think in these terms, it would be of great interest to many of us if you would explain the contradiction such that we can discuss it -- hopefully learning something along the way.
E.g., you said that the difference in noise characteristics between different sensors is to be found elsewhere (than the total light captured). Please explain where and why. Or you said that Frank should find the contradiction in his argument. Please point it out, I think the discussion would greatly benefit.
-
Frank has to answer for himself. His example had logical flaws in it.
As I said, now several times and phrased differently, yet obviously to no avail, I find these discussions so utterly worthless. We commenced with the false assertions of the "crop factor" thinkers and now obviously have entrenched ourselves in the borrows of the "equivalence" creationist. To what use?
Photography is not about "equivalence" and "sameness", it is about visual expression and being able to previsualise ideas. I have no clues as to why this is so difficult to understand unless photography as an art is dead and technical exercises replace it.
Added for clarity:
Just to make sure about the basic facts: formats are different. They always have been in their behaviour and technical solutions from the film days. Larger formats have less need of secondary magnification and hence show less grain (on a relative scale). Lenses for these formats resolve less because they must be designed to cover a larger area. The danger of empty magnification is small. They operate slower. A smaller format needs more secondary magnification, hence is prone to show more graininess. On the other hand, their lenses can be made faster and with higher resolving power thanks to their required image circles being smaller. The issue of empty magnification becomes more real. The systems operate faster.
For every different photographic application there is a system that optimises the specific requirements. This optimum would not be constantly placed on the "format scale". Acceptance over time for smaller format grew over time without being universally accepted.
Digital technology does not change these facts. What we have seen is, however, that the superior technology of digital can push the relative relationships to cover smaller physical formats than before, but sooner or later the physical laws and the requirements for secondary magnification, dynamic range, etc. introduce limits that lead to less than desirable performance. However, still the balance point is up to the individual photographer and their assignment, and no point on the "format scale" can be considered permanently fixed as the demands do change.
Now, this is the *end point* as far as I am concerned.
-
The only question that remains for me ... I generally am 100% consistent with Simone Surace ...
is: from the same perspective the D500 + 1.4/24 shot still looks different from the D600 + 1.4/35 shot
although I did everything to have same
perspective
format adapted time and aperture
field of view
The difference I see is in the BG rendering.
Does the focal length play in here?
Or is it the lens characteristics?
Have to leave. More later.
-
Actually the noise itself is higher for the larger format. The noise varies as the square root of number of photons which means that the signal-to-noise ratio (SNR) varies in the same way. So the larger format has a higher SNR and it is this which manifests itself in a "less noisy" picture.
I mention this not in the cause of pedantry but because Eric Fossum made the same point [...]
Another way of putting Eric Fossum's point is that you can treat the whole sensor as a single pixel, and the bigger the pixel the lower the noise. You can do that because it turns out that you can bin pixels into larger and larger units with no increase in noise. The problem with that analysis is that it does not tell you anything helpful about images with significant amounts of information content.
In any image there are two sources of variation in voltage between pixels: noise and true pictorial information (noise as false pictorial information caused by lens aberrations could be considered separately as well). The only practical way to identify noise is to eliminate pictorial information by photographing a uniformly illuminated screen, so you can assume that all pixel to pixel variation is noise. If you are not interested in pictures of uniformly illuminated screens the SNR is not the square root of the number of photons. The signal includes information about the picture, and you can't bin pixels without losing information about the picture. A single pixel has an information signal to noise ratio of almost zero: the only information you get is the average scene luminance.
Smaller pixels allow you to collect more content information, because the sampling rate is higher. Smaller sensors have more noise than bigger sensors, but the pictorial information the smaller sensor collects may be less, the same or more, depending on the pixel density (and on the quality of the lens), so the signal to noise ratio including pictorial information in "signal" can be anything.
In most photographs the pictorial information is overwhelmingly predominant as a component of "signal" compared to the average scene luminance. So sensor size has no important effect on the signal-including-pictorial-information to noise ratio, except when there is little pictorial information so that average luminance is a large proportion of the "signal" (eg, pictures of blue sky or areas of still water reflecting the sky).
Of course, there is an additional component of "signal": aesthetic or, as the case may be, political or scientific information. If it is present, as it all too often is not, that is so much larger a component of "signal" than any other that sensor size becomes completely irrelevant.
-
Well, what you now describe has absolutely nothing to do with the starting point of this thread.
If we speak equivalence we have to have equivalent lenses and sensors. That is my point.
I do not answer to the starting point but to the flow of the discussion.
-
The difference I see is in the BG rendering.
Does the focal length play in here?
Or is it the lens characteristics?
May I note that the size of the aperture (not the aperture ratio) is larger for the 35/1.4 lens than for the 24/1.4 lens. It's my understanding that when the background is well outside the DoF zone the lens with the larger aperture (not the aperture ratio) will give more background blurring.
How Much Blur: 105/2.8 FX v. 70/2.8 DX v. 70/2.0 DX (http://howmuchblur.com/#compare-1x-105mm-f2.8-and-1.5x-70mm-f2.8-and-1.5x-70mm-f2-on-a-0.9m-wide-subject)
How Much Blur: 35/1.4 FX v. 24/1.4 DX (http://howmuchblur.com/#compare-1x-35mm-f1.4-and-1.5x-24mm-f1.4-on-a-0.9m-wide-subject)
Bokeh and Background Blur - Bob Atkins (http://www.bobatkins.com/photography/technical/bokeh.html)
Dave
---
Edit 2016-07-18: didn't want to bump the thread.
[I guess I should have included this link but I didn't because the calculator offered does not work with Windows7 and later. While the calculator doesn't work some of the text is worth reading. Here is the link...
Background Blur and Bokeh - a multifunction optical blur calculator (http://www.bobatkins.com/photography/technical/bokeh_background_blur.html)
I'll quote one paragraph...
"One thing that this calculator demonstrates, and it's a point that people often fail to realize, is that depth of field and background blur are not the same thing and indeed may not even be closely related. People think that a lens setting with a small depth of field will blur the background more, but that isn't necessarily true. The math is quite different in the two cases. Depth of field will give you an estimate of local blurring about a subject in focus, i.e. how blurred the image will be just outside the traditional "depth of field" limits. However that doesn't correlate with how blurred the image will be of objects at a significant distance behind the subject in focus." --Bob Atkins.]
-
Added for clarity:
...
I agree with everything you write in your clarification, since it is logically equivalent to what I have written earlier.
The important thing is that something changes between formats, and how that thing scales with the size of the sensor. Whether you call it 'the secondary magnification that is required to produce the same output' or something else does not matter. What matters is the set of predictions derived from the argument, and whether those predictions turn out to be correct. It is only because a digital file loses its physical dimension whereas a negative does not that some people now prefer not to talk of secondary magnification. So they use different words to describe the same fundamental relationships.
Of course many people misunderstand this stuff. But they would likewise misunderstand any other terminology, even more if they never worked with film formats. The misunderstandings won't go away by abolishing a terminology, but hopefully they will diminish by being explained.
Please don't call this debate 'creationist'. First, you are putting people who understand the stuff and those who don't in the same bin just because they are using the same terms, which creates more confusion among those that don't understand it. Secondly, we are engaging in a rational/scientific discussion on a technical topic, which is quite unlike 'creationism'.
I'm also aware that this discussion has very little to do with the artistic aspects of photography, but this is not the point here. My aim is to create clarity about the technical stuff. This discussion relates to photography in the same way that a discussion of chemical reactions relates to culinary arts. It is up to the individual photographer/cook to decide how important it is or whether they want to use these insights in their daily work or to guide their purchasing decision, but the truth or falsehood of it is independent of this individual decision.
-
Thank you Simone.
Now I will extract the original files of the "practical (non)equivalens group shot" from one of my backup disks and make them available to download
-
May I note that the size of the aperture (not the aperture ratio) is larger for the 35/1.4 lens than for the 24/1.4 lens. It's my understanding that when the background is well outside the DoF zone the lens with the larger aperture (not the aperture ratio) will give more background blurring.
How Much Blur: 105/2.8 FX v. 70/2.8 DX v. 70/2.0 DX (http://howmuchblur.com/#compare-1x-105mm-f2.8-and-1.5x-70mm-f2.8-and-1.5x-70mm-f2-on-a-0.9m-wide-subject)
How Much Blur: 35/1.4 FX v. 24/1.4 DX (http://howmuchblur.com/#compare-1x-35mm-f1.4-and-1.5x-24mm-f1.4-on-a-0.9m-wide-subject)
Bokeh and Background Blur - Bob Atkins (http://www.bobatkins.com/photography/technical/bokeh.html)
Dave
Nono, this is not what I meant. For the group shot I shoot FX at f=11 and DX at f=8 as stated in the theoretical formula given by physics.
My question aims at the:
1) drawing (lens geometry and other (non)corrections)
2) background compression (more tele in 35 than in 24)
-
May I note that the size of the aperture (not the aperture ratio) is larger for the 35/1.4 lens than for the 24/1.4 lens. It's my understanding that when the background is well outside the DoF zone the lens with the larger aperture (not the aperture ratio) will give more background blurring.
How Much Blur: 105/2.8 FX v. 70/2.8 DX v. 70/2.0 DX (http://howmuchblur.com/#compare-1x-105mm-f2.8-and-1.5x-70mm-f2.8-and-1.5x-70mm-f2-on-a-0.9m-wide-subject)
How Much Blur: 35/1.4 FX v. 24/1.4 DX (http://howmuchblur.com/#compare-1x-35mm-f1.4-and-1.5x-24mm-f1.4-on-a-0.9m-wide-subject)
Bokeh and Background Blur - Bob Atkins (http://www.bobatkins.com/photography/technical/bokeh.html)
Dave
What I'm seeing in Frank's comparison are geometric differences, which I would attribute to the lens distortion characteristics and the potentially different position of the entrance pupil between the two shots, which creates a different perspective. Also, the angle of view is only approximately equivalent.
-
Full size.
Full EXIF.
Sorry, both developed in Photo Ninja.
Results would be much better with NX-D.
Instead of f=5.6 (DX) and f=8.0 (FX) I accidentially dialed in 5.6 & 11.
http://fotokontext.de/ZENTRALKRAFT/email_ANP_9908_v1.JPG
http://fotokontext.de/ZENTRALKRAFT/email_DSC_1107_v1.JPG
-
Thanks, Simone, I am not so much seeking equivalence, but trying to get a view of what changes with changes in equipment. Your comments help me.
-
What I'm seeing in Frank's comparison are geometric differences, which I would attribute to the lens distortion characteristics and the potentially different position of the entrance pupil between the two shots, which creates a different perspective. Also, the angle of view is only approximately equivalent.
real world lenses neither have exactly the focal length stated (not even at infinity **ggg**) nor are they perfectly corrected for geometry and all other drawing characteristics.
That is why I write "equivalens" not "equivalence"
The theory is good, but is has not much practical meaning.
For day-to-day work the competent use of perspective is much more important.
One of the first things (just after: tell me where the light sources are) I teach to people coming to me to learn photography basics from me is, that perspective distortion is independent of camera and lens!
So I suggest to keep the same distance (perspective) in portraiture, roughly 2.5 to 3.5 meters and choose focal length just to add or leave out context.
I can easily show the "big nose" and "bended head" effect of a perspective too near to the subjects.
-
difference between the pictures
the magnification (21MP vs. 24 MP) and (35mm and 24mm) and (FX vs. DX) is normalized to the kids in the first row
verdict:
1) I will do the test again under more controlled conditions.
2) From what I see the group is more or less rendered the same modulo subject movement, the BG looks much different though
-
May I note that the size of the aperture (not the aperture ratio) is larger for the 35/1.4 lens than for the 24/1.4 lens. It's my understanding that when the background is well outside the DoF zone the lens with the larger aperture (not the aperture ratio) will give more background blurring.
It is not the aperture, it is the magnification. The key is to distinguish absolute and relative blur. Absolute blur is the size of the blur circle of a point on the print or screen, and that, of course, is greater for lenses of longer focal length. If you normalise for image magnification you have relative blur and that depends on F number. There is a good example at http://toothwalker.org/optics/dof.html - scroll down to images 3 and 4.
-
difference between the pictures
the magnification (21MP vs. 24 MP) and (35mm and 24mm) and (FX vs. DX) is normalized to the kids in the first row
verdict:
1) I will do the test again under more controlled conditions.
2) From what I see the group is more or less rendered the same modulo subject movement, the BG looks much different though
The point is that you managed to produce almost the same shot using two different formats. It is possible using the correct settings. There are minor differences but they would not matter to anyone unless they could choose between the two images. You could have done a similar experiment using a cell phone and a medium format system. At some point you will run into limitations, and this is where the right choice of format becomes important, i.e. it actually matters.
-
And: Is "almost the same shot within given limitations" = "equivalence" ???
-
And: Is "almost the same shot within given limitations" = "equivalence" ???
No, equivalence is simply the idea that you can in principle set the two cameras in a way that ensures that certain parameters are the same. In practice, there are many caveats of course, but they are exactly the things that constrain your choice of format/equipment/system (e.g. not being able to choose a certain lens).
Why don't you do your formal group shots with a cell phone? It is because you can't get an equivalent shot as with your DSLR, or the equivalent shot puts you in a setting where you are not comfortable and where you don't need to go if you use your DSLR.
-
It is not the aperture, it is the magnification. The key is to distinguish absolute and relative blur. Absolute blur is the size of the blur circle of a point on the print or screen, and that, of course, is greater for lenses of longer focal length. If you normalise for image magnification you have relative blur and that depends on F number. There is a good example at http://toothwalker.org/optics/dof.html - scroll down to images 3 and 4.
If you are interested in how big the bokeh balls are relative to the imaged subject (as opposed to the scale of the blurred object), then it is indeed the absolute size of the entrance pupil that matters -- irrespective of magnification or focal length (assuming that the point light source is far away and that the angle of the parallel rays to the optical axis is small).
In the gromit example in your link, the bokeh balls of the 100mm/4 shot are bigger than in the 28mm/4 shot when measured relative to the gromit. The sizes of appear roughly as 25mm and 7mm measured in the plane of the gromit. These are the respective sizes of the entrance pupils.
-
Alongside probably many on this forum, I've been following this thread with interest, and learnt how confused I have been about the fundamentals of optics WRT different film formats/sensors. And I am not alone in it being decades since I gave due thought to the attributes of lenses (as demanded in physics in high school and 1st year university). Nothing in my personal photography library touches on this stuff, and even some academic ebooks I checked this weekend are either superficial and/or vague, or far too technical. So I'm sure I'm not alone in having already learnt new and important insights within the thread. So thanks for inputs, and honest questions
And I agree fully only science can sift fact from myth, yes, our aim "is to create clarity about the technical stuff" and it's up to the individual how they work with such insights.
It will be most valuable as resolution consolidates :-) to summarize the axioms in point form
kind regards
Woody
I agree with everything you write in your clarification, since it is logically equivalent to what I have written earlier.
The important thing is that something changes between formats, and how that thing scales with the size of the sensor. Whether you call it 'the secondary magnification that is required to produce the same output' or something else does not matter. What matters is the set of predictions derived from the argument, and whether those predictions turn out to be correct. It is only because a digital file loses its physical dimension whereas a negative does not that some people now prefer not to talk of secondary magnification. So they use different words to describe the same fundamental relationships.
Of course many people misunderstand this stuff. But they would likewise misunderstand any other terminology, even more if they never worked with film formats. The misunderstandings won't go away by abolishing a terminology, but hopefully they will diminish by being explained.
Please don't call this debate 'creationist'. First, you are putting people who understand the stuff and those who don't in the same bin just because they are using the same terms, which creates more confusion among those that don't understand it. Secondly, we are engaging in a rational/scientific discussion on a technical topic, which is quite unlike 'creationism'.
I'm also aware that this discussion has very little to do with the artistic aspects of photography, but this is not the point here. My aim is to create clarity about the technical stuff. This discussion relates to photography in the same way that a discussion of chemical reactions relates to culinary arts. It is up to the individual photographer/cook to decide how important it is or whether they want to use these insights in their daily work or to guide their purchasing decision, but the truth or falsehood of it is independent of this individual decision.
-
Phewww, I start a very clear thread and come back few days later to find this:
"Round, Like a circle in a spiral
Like a wheel within a wheel,
Never ending or beginning,
On an ever-spinning reel"
My thread is very simple, if you mount the same focal lens on an Fx camera or Dx camera you still have the exact focal lenght.
Your camera or lens does not become a factorial of the cropped sensor. All what is being done is a CROP.
I also mentioned as a "fail safe" point that people who like to be anal about the statement, will find a "minute" difference in the DoF.
Par Bjorn and like minded people, It seems that the Anal's are taking over the asylum with ad nauseum half baked optical and physics refer3ences.
THE FACT OF THE MATTER IS THIS:
1- Fx provides a better picture than Dx, you are free to understand a (better picture) in any which way you like sunny side up.
2- In fact Dx existence is a product aberration in as so much to have an answer to Canon at the time. Which resulted in having a camera at a lower price point than FX.......that's it.......it is a money issue disguised as a marketing exercise.
3- I do not know how many on this board are versed in design or painting in the sense of having learned and practiced design and painting where Perspective is your Vista and Color Harmony is your palette.
Most importantly what you learn in Design and Painting and Photography is the following:
"IF IT IS GOOD TO YOUR EYE, IT IS GOOD TO GO"
and that is why and how I am what I am today in the Art and Photography industry.
My eternal thanks to my Design and Cinematography instructors.
I will leave you with this:
https://www.youtube.com/watch?v=7VydfUohuaM
.......or indulge in continuing the thread to your liking........
-
I agree with the above, except to remember that all formats are a product aberration. FX is arbitrary too, depending on the size of 35 mm. film, which is in its turn arbitrary, and so forth. The native superiority of one format over another is relevant only as long as improvements are applied to it. None of the refinements that turned the terrible, low fidelity audio cassette into a listenable medium were unavailable to other formats, but for various reasons not directly related to native audio quality they were not applied.
-
Thanks Simone for writing so clearly about the topic. I think eventually, photography textbooks should be rewritten to better reflect current technology and the physics of imaging. With film, sensitivity was largely fixed and most film types were available in only one or two speeds, and the photon detection wasn't as efficient as it is today. Photographers would use film and they'd know that fast film yields grainier images, but most didn't really stop to think why it is so. It's because recording only a small amount of light leads to uncertainty in the measured quantities because of the randomness of the arrival times of individual photons. This is the not the complete picture (there are other sources of noise which fortunately tend to become less significant as technology matures) but it's the one factor which cannot be removed by any advance in technology, thus it is important to understand. Today we have sensors where the ISO can be adjusted across a huge range and the image quality (in terms of SNR) closely approximates the limits of what is theoretically possible. This has meant that the imaging systems can be now understood using a theoretical framework which considers relatively simple physical principles first, and leaves the complex implementation details out (at least in the first approximation), yet this theoretical analysis gives useful results which are close to what is experienced in real life using these new systems. Hence the models can be helpful to aid understanding how different cameras can be used to create similar results (certain things are not dependent on the imaging system but stem from the fundamentals of the physics of imaging and optics) and also to elucidate the underlying physics of why results using given settings and setups are different.
I can understand that a photographer working in practice needs to use the tools that are in hand and cannot replace them with tools that might exist only in theory. However, it cannot hurt to try to understand basic principles in different ways. Richard Feynman was famous for his ability to come up with different analogous models to explain physical and mathematical phenomena. This kind of thinking is very useful and deepens one's understanding. Of course, none of these explanations is really the "why" of how things work the way they work in nature. That kind of question cannot be answered. What we know is certain basic principles such as the conservation of momentum can be used to explain and predict a lot of physical phenomena and these principles are widely used in engineering. It should be the same with photography. The usefulness of a model or theory is validated by applying the model or theory to predict the result of a new experiment.
Today many sensors are close enough to the "ideal sensor" that simple principles can be applied with great success, as shown by the dpreview.com article on equivalence. There is no "snake oil" being sold here. Equivalence predicts, for example, that if the camera position, shutter speed, angle of view, and depth of field are set the same, then the signal-to-noise in the final image will be about the same as well in the final print (of the same size, viewed from the same position), independently of the sensor size. This is a very important result and an important validation of the usefulness of the theory. It is helpful because it tells us as photographers that at diffraction limited apertures, noise is really a function of how much depth of field you want, not a question of which format you choose to work with. I'm assuming here that subject movement (e.g. due to wind or another reason) forces us to use a certain shutter speed and thus shutter speed is not a parameter we can play with. Thus a macro photographer who needs reasonable depth of field can work with whatever format they find the most practical and will not be punished in terms of image quality until the lens becomes aberration limited instead of diffraction limited. Another, interesting result from theory is that if we set depth of field so that the lens is diffraction limited, at equal depth of field, all formats suffer the same amount of blur due to diffraction. Essentially small formats are very good tools for photographing small subjects whenever you need a lot of depth of field. Larger formats can give the same results but they are larger and more expensive. However, what favours larger formats are the aberrations at wide apertures. For shallow depth of field work, larger formats are preferable because relative to image size the aberrations play a smaller role (at least in central areas of the image) and you get to choose from a larger range of apertures which give "good enough" detail in the final image. All of these results can be understood by theory and are confirmed by practical experience.
However, in certain cases an individual tool has a distinct advantage. For example there is no DX equivalent to the 24/1.4 on FX, a lens that would give a similar picture on DX as that lens at its maximum aperture gives on FX. Another thing is that if you are not comparing equal shutter speed cases, then you may find that one tool works better. For example there is no DX equivalent to a D810 set to ISO 64. Not in terms of resolution of the final image nor in the dynamic range. But these are "fringe" areas where the implementation of a particular camera is unique and offers an advantage. Similarly a DX camera can give more detail in good light than an FX camera using the longest lens you have. Thus not everything can be replaced by an equivalent something else.
-
Simone: As far as I can see, this topic is not about photography specifically. It is about optics and digital sensors....There is no artistic aspect to this topic as far as I can see. It is purely technical/scientific.
Exactly.
******
I'm just not sure what the confusion is about equivalence?? (Or whatever one wants to call it.)
What is it that people are not understanding? Perhaps there is/was simply a confusion in the terminology??
There are several clear explanations in this thread.
And there is a really good link to detailed explanations.
-
If you are interested in how big the bokeh balls are relative to the imaged subject (as opposed to the scale of the blurred object), then it is indeed the absolute size of the entrance pupil that matters -- irrespective of magnification or focal length (assuming that the point light source is far away and that the angle of the parallel rays to the optical axis is small).
That is true of specular highlights. Here are two images, taken from the same place, one with a 50mm at f/5 and one with an 85mm at f/8, so (roughly) the same absolute aperture. Both lenses were focussed at 0.8m. The specular highlights are indeed the shape of the aperture and (roughly) the same size. But areas other than specular highlights - the fence railings, eg - scale with magnification.
-
I'm just not sure what the confusion is about equivalence?? (Or whatever one wants to call it.)
What is it that people are not understanding? Perhaps there is/was simply a confusion in the terminology??
There are several clear explanations in this thread.
And there is a really good link to detailed explanations.
Andrea B. I feel the thread is much about Physics for everybody but the thread opener. That is why the nice people here taking some effort to rephrase the issue over and over again, ignore the original impuls so elegantly.
Love, Frank
-
That is true of specular highlights. Here are two images, taken from the same place, one with a 50mm at f/5 and one with an 85mm at f/8, so (roughly) the same absolute aperture. Both lenses were focussed at 0.8m. The specular highlights are indeed the shape of the aperture and (roughly) the same size. But areas other than specular highlights - the fence railings, eg - scale with magnification.
I'm not saying the balls don't scale with magnification. I'm just saying that their size relative to the subject in the plane of sharp focus does not scale with magnification. I don't see why the behaviour should be different for point sources and for points of a railing. All light rays behave the same.
My understanding is that if I want to design a shot with a 5cm flower (which is in focus) and a bokeh ball such that the flower is the same size as the bokeh ball in the final image, I need a lens that has at least an entrance pupil of 5cm, no matter how big the flower and the bokeh ball will appear in the final image. Thus, I could take this shot with a 300/4 lens (stopped down slightly) but not with a 50/1.4 lens. The size relationship of the flower and the ball will be the same regardless of the distance to the subject provided that the light source that produces the bokeh ball is at infinity. If I focus and move closer to the flower, I will have an image with a big flower and a big ball, if I move further away, both the flower and the ball will become smaller until they both become close to a point if I'm on the other side of the solar system.
(This is easy to see if one draws a diagram of the marginal parallel light rays coming from the point source (the rays that just make it through the aperture) and thinks of the intersections of these rays with the subject plane as the margins of an imaginary planar light source. The physical size of this source is the same as the size of the entrance pupil.)
-
I'm just not sure what the confusion is about equivalence??
One thing that keeps popping up over and over again is that equivalence theory apparently contradicts the fact that external light meters work (taking issue with the theory placing more importance on total light vs. exposure). This is of course a false assertion. Light meters do work and they give you a reading in terms of exposure that is applicable to all formats in the same way. It is nevertheless true that total light has a certain importance depending on how you look at the whole situation. There is no contradiction. The important questions are: how important is exposure really? How should we expose our sensors to get optimal data? Has anything changed from film to digital in the way we expose?
Most of the other issues that people have are hard for me to identify. We need people to explain them more clearly.
-
The important questions are: how important is exposure really? How should we expose our sensors to get optimal data? Has anything changed from film to digital in the way we expose?
The first question is surely : what do we mean by "exposure" ? The word is used in at least two ways, often by different people in a single conversation, and confusion can arise. My preferred usage is that from Wikipedia ( https://en.wikipedia.org/wiki/Exposure_(photography) ) : In photography, exposure is the amount of light per unit area (the image plane illuminance times the exposure time) reaching a photographic film or electronic image sensor, as determined by shutter speed, lens aperture and scene luminance. However what then the role of ISO and the "exposure triangle", where ISO is given an equal importance with aperture and shutter speed ? To my mind, the exposure triangle describes output "brightness" rather than exposure - but many would disagree. The subject has received much attention on DPRev recently.
We are often told that exposure is fundamental and doesn't change from when moving from film to digital. However the device (film or electronic sensor) response to light differs. For film we have the characteristic "S-shaped" Hunter-Driffield or "characteristic" curves. The digital sensor is largely linear. So actions like "expose for the shadows and develop for the highlights" - or is it the other way round - might be useful for film but not for the electronic sensor. For the electronic sensor there is nothing to be gained by not collecting the maximum amount of light. So shutter speed is chosen to minimise movement blur - or maybe to allow it, aperture is chosen for DOF and/or the optimum performance of the lens and the natural scene illuminance is accepted - or maybe supplemented with flash. Exposure, as defined by Wiki is then determined. Knowledge of the sensor read noise as a function of ISO enables the optimum camera setting. If a picture is not bright enough, or too bright, perhaps people could say so without referring to "underexposure" or "overexposure".
-
I'm not saying the balls don't scale with magnification. I'm just saying that their size relative to the subject in the plane of sharp focus does not scale with magnification. I don't see why the behaviour should be different for point sources and for points of a railing. All light rays behave the same.
My understanding is that if I want to design a shot with a 5cm flower (which is in focus) and a bokeh ball such that the flower is the same size as the bokeh ball in the final image, I need a lens that has at least an entrance pupil of 5cm, no matter how big the flower and the bokeh ball will appear in the final image. Thus, I could take this shot with a 300/4 lens (stopped down slightly) but not with a 50/1.4 lens. The size relationship of the flower and the ball will be the same regardless of the distance to the subject provided that the light source that produces the bokeh ball is at infinity. If I focus and move closer to the flower, I will have an image with a big flower and a big ball, if I move further away, both the flower and the ball will become smaller until they both become close to a point if I'm on the other side of the solar system.
In that case I don't understand what you mean by "bokeh ball". If you mean the image of an isolated (AKA "specular") highlight then no: they do not get smaller as you move away precisely because they are an image of the aperture, which is the same size however far away you are from the subject. If by bokeh ball you mean the image of something roundish that has lost all form because it is far out of focus then its size does change in step with the in focus area, regardless of the aperture. Here is an example: 85mm at f/4, focused at 0.8m, from nearer and further. The specular highlights are the same size, but the other features of the car are smaller as you get further away.
-
In that case I don't understand what you mean by "bokeh ball". If you mean the image of an isolated (AKA "specular") highlight then no: they do not get smaller as you move away precisely because they are an image of the aperture, which is the same size however far away you are from the subject. If by bokeh ball you mean the image of something roundish that has lost all form because it is far out of focus then its size does change in step with the in focus area, regardless of the aperture. Here is an example: 85mm at f/4, focused at 0.8m, from nearer and further. The specular highlights are the same size, but the other features of the car are smaller as you get further away.
You are correct, however in my example I implied a refocus with changing distance (to get the flower sharp at the new distance). The ball I was talking about was from a point light source at infinity and not part of the main subject. Your examples don't show this because firstly, you don't have anything sharp to compare the scale of the ball to, and secondly, you did not move your focus ring between the two examples. Of course the ball gets smaller as you focus further away, which was my point, and my second point was that the subject will get smaller by the same factor because you move further away as you refocus, keeping the ratio of their sizes the same throughout. I don't think we disagree here.
-
Exposing for the shadows (when using negative film) implies metering shadow areas, and ensuring they are given enough exposure. That means giving more exposure (not less) than exposing for the highlights.
-
Exposing for the shadows (when using negative film) implies metering shadow areas, and ensuring they are given enough exposure. That means giving more exposure (not less) than exposing for the highlights.
Thank you. You reveal my ignorance in using film. I don't know much about it because I never used it. And yet, when I started taking photographs using a digital camera, many film users were trying to use their existing film techniques for digital. I'm suggesting that this is not always appropriate. You may wish to select the linear region of the light-response curve of film, but in a digital sensor the response is almost wholly linear - except maybe near saturation.
-
Les,
To my eye the blur that is the tail lights of the car in your second photograph has become as big as the blur from the specular light source.
My interest is having the background blurred to the extent that items in the background aren't particularly recognizable so they do not compete with the subject and having a sharp subject against a softly blurred background makes the subject "pop" as is frequently said. Also for some subjects I don't want to sacrifice my preferred or chosen perspective so I don't want to use a longer lens on a small format to gain a larger entrance pupil.
The first time I tried a 105/2.5 Nikkor-P on my Nikon F I loved the look. The best I can do on DX is use an 85/2.0 and backup and have a flatter perspective than I want.
Dave
-
Les,
To my eye the blur that is the tail lights of the car in your second photograph has become as big as the blur from the specular light source.
My interest is having the background blurred to the extent that items in the background aren't particularly recognizable so they do not compete with the subject and having a sharp subject against a softly blurred background makes the subject "pop" as is frequently said. Also for some subjects I don't want to sacrifice my preferred or chosen perspective so I don't want to use a longer lens on a small format to gain a larger entrance pupil.
The first time I tried a 105/2.5 Nikkor-P on my Nikon F I loved the look. The best I can do on DX is use an 85/2.0 and backup and have a flatter perspective than I want.
I can't tell where the tail lights begin and end in either case. The exposures are slightly different - 1/80 in one case and 1/125 in the other because more of the brightly lit foreground is included in the wider shot so the car itself is slightly darker.
However, the specular highlights are the size of the aperture and do not change in size with distance. Obviously, the image of the car is smaller when the camera is further away. So the specular highlights are bigger relative to the car when the camera is further away. If the blur zones of non-highlight areas - the line between the white of the car and the shadow underneath the car, eg - behaved the same way as the highlights the apparent blurriness of the image would increase as the camera moved further away, and it doesn't.
An alternative to an 85mm on DX (diagonal aov 19 degrees) and more distance as an approximation to 105mm on FX (diagonal aov 23 degrees) is a 58mm on DX (diagonal aov 27 degrees) and a discreet crop or a slightly smaller distance for framing and an adjusted aperture for DoF. But unless you are placing the subject very close to both edges of the image, I don't see that 1-2 degrees at either side is photographically significant. (People who don't have these lenses can simulate the difference: your hand with thumb and fingers spread and held at arm's length subtends about 20 degrees and your index finger at arm's length is 1 degree).
-
Les,
58mm is even shorter than 70mm so the entrance pupil is smaller in spite of being an f/1.4 or f/1.2. I own a 50/1.2, 50/1.8 and 60/2.8 and also a paper weight AF 70/2.8D Nikkor with a decentered element or group. None of these gives me what I have with a 105/2.5 AIS in terms of background blurring with a subject at 2m or so and a more distant background. For me the easy choice is FX (24x36mm).
Dave
-
I feel it is very helpful at one stage of consciousness to bring up to awareness all the things you did do right or wrong by
intuition earlier on the path of the photographer.
One usually begins with an awareness of the main subject and is happpy if that is pictured more or less the way
you wanted it to be pictured.
Also one camera with one lens is usually the starting point.
So one learns about and tries the effect of aperture and time on Depth Of Field and Motion Blur.
That Makes Three: Getting the exposure right and at a certain exposure decide which combination of time and
aperture are just right. Both still centered on the main subject. Like in: choose f=11 or f=16 for Architecture and f=2 to f=4
for Portraits.
Then, that brings us a big step forward: the young photographer discovers the background as a very important
element of consideration. But that is the STARTING POINT.
Because. The BACKGROUND is not only form and area and color dissolving or being definitely rendered adding
to or distracting from the main subject.
The background comments on or plays a supporting role in the story you tell in the picture.
The background rendering changes with focal length and rendering style of lenses which increases the interest in
more and more different lenses....
That is where this discussion kicks in ...