Lenses focal length

[ArendV]

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).

[Andrea B.]

Arend  .
Didn't Sigma offer a firmware fix? I thought I read that somewhere.
 
Yes, the James article is a good write-up I think.

[chambeshi]

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.

[chambeshi]

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.  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.

[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 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.

[David H. Hartman]

Dynamic Range v. ISO for my D300s and D800 in both DX and FX modes.

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

[ArendV]

@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.

[Bjørn Rørslett]

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.

[Frank Fremerey]

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....

[Bjørn Rørslett]

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?

[pluton]

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. 

[Bjørn Rørslett]

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.

[Roland Vink]

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.

[Bjørn Rørslett]

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

[Andrea B.]

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.