Lenses focal length

[David H. Hartman]

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

[ArendV]

Light intensity is the same,  the total amount of light captured is lower on DX vs FX.

[Les Olson]

"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. 

[simsurace]

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.

[bjornthun]

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. 

[Bjørn Rørslett]

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.

[ArendV]

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.

[Thomas Stellwag]

to all:  just wait a second - i need to buy popcorn

[Frank Fremerey]

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.

[Frank Fremerey]

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)

[Andrea B.]

If anyone wants a sensible discussion of "equivalence", then I recommend the following:  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.

[Asle F]

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.

[Andrea B.]

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? 
(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.]

[ArendV]

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.

[Ilkka Nissilä]

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.