Discussion of 'Equivalence'

[simsurace]

No.  The D500 has 20MP.  At 300 dpi, 8 x 10 is 5.4MP, 12 x 18 is 9MP, 24 x 36 is 18MP.  Because I have spare pixels, I can print all those sizes at identical resolution: there is no "enlargement" of the DX image as output size increases until I exceed 24 x 36.  It was different when a D2 only had 4MP: then printing larger was the same as enlarging a negative, because except for tiny prints the only way to print bigger was to lower dpi.   

A D810 has 36MP and a D500 has 20MP, so the sensor elements are slightly smaller on the D500 (pixel pitch 4.2 microns vs 4.9).  Suppose I have the same lens on both: the same object is imaged the same size on both sensors. That image covers 1.2 (4.9/4.2) times as many sensor elements on the D500 sensor as on the D810 sensor.  If I print the D500 and the D810 images at 300 dpi, the printed objects are 1.2 times larger (not 1.5). But I can print the D500 image at 360 dpi instead of 300 dpi, so that the printed images are the same size for both formats (360/300 = 1.2).  I cannot quite manage it at 24 x 36, which at 360 dpi is 21.6MP, but if I am allowed to adjust print resolution (and you just try and stop me  ) there is no sensor "enlargement" except at the most extreme print sizes.  For the D5, which has 6.4 micron pixel pitch, I would have to print at 450 dpi instead of 360 dpi to eliminate enlargement (6.4/4.2 = 1.5).   

Remember that the 'secondary magnification' argument was brought up by Bjørn as an alternative explanation for why images from different formats behave differently. So you are now disagreeing with Bjørn, not me, about the applicability of this concept to digital images. I think it would be best if he himself responded to that.

[simsurace]

So, let me set a problem for equivalence.  I want to reproduce the look of David Bailey's 1965 portrait of the Kray twins, Reggie and Ronnie (they were violent criminals, Reggie is on the left, Ronnie on the right; Bailey had grown up in the same neighbourhood and knew them; "I quite liked Reg, even though when he was 19 he slashed my father’s face with a razor. Ron was a basket full of rattlesnakes." https://www.theguardian.com/artanddesign/2015/jul/05/david-bailey-stardust-exhibition-edinburgh-photographer-interview - the whole interview is well worth reading).  The portrait was made with an 80mm f/2.8 on 6 x 6 film.  Does anyone think that knowing what would be equivalent on FX or DX is where I need to start?

Nice iconic photograph. No, I don't think that is where you would start. I would first try to find some people/actors that can deliver this kind of impression. But at some point, a consideration of focal length and camera position have to come into play if I'm to reproduce the same perspective and framing.

Now, what is your point?

[Les Olson]

My point is that "equivalence" is not helpful in making photographs.  All I need to know is that 80mm is "normal" on 6 x 6: then I know that I need a "normal" lens - on FX 35mm to 50mm and on DX 24mm to 35mm. There are plenty of lenses in the right range, for FX and DX.  The reason I want that unusual - for portraits - focal length is that I want the unusual perspective, which contributes to the intimidating atmosphere, but I do not need to know exactly how far away Bailey was: I will judge how close to get as I look in the viewfinder, which will depend on, eg, exactly how big my subject's shoulders are, which won't be the same as Ronnie Kray's.  The lighting, the expressions, the clothes and the low point of view are all format independent, and DoF is quite deep, so that is not a problem in any format.   

I do not need to know the "equivalent" focal lengths because I do not want "the same framing and perspective" because I am not trying to copy Bailey's image.  "Equivalence" is only helpful if you are trying to copy images, and copying images is pointless - as well as, in the case of other people's images, not obviously legal. 

If it were answered that in this case the DoF is quite deep, but in others I might want a very shallow DoF, I would ask for an example by an important photographer of an image depending on DoF so shallow that DX could not approximate it.  Very shallow DoF is not, contrary to the impression discussions of equivalence give, a widely used pictorial device; here is an example of creative use of unusually shallow DoF, by Sudek, from Labyrinths, but it is not beyond the reach of DX with ordinary lenses.

Do I really need to spell out an argument for why, if "equivalence" is not helpful in making photographs, it is not helpful in evaluating equipment for making photographs?
 

[simsurace]

My point is that "equivalence" is not helpful in making photographs.  All I need to know is that 80mm is "normal" on 6 x 6: then I know that I need a "normal" lens - on FX 35mm to 50mm and on DX 24mm to 35mm.   

That is, loosely speaking, a statement about equivalence of focal lengths regarding angle of view (AOV). You are arguing against something that is so deeply ingrained in your thinking that you don't even notice it. All that James and others did was not to stop at AOV, but extend the concept to other important image characteristics.

I do not need to know the "equivalent" focal lengths because I do not want "the same framing and perspective" because I am not trying to copy Bailey's image.  "Equivalence" is only helpful if you are trying to copy images, and copying images is pointless - as well as, in the case of other people's images, not obviously legal. 

In your statement above, you already determined equivalent focal length ranges on several formats. You just used a very high tolerance in terms of the resulting FOV.
 
Whether you want to copy the image exactly or just imitate it roughly is a matter of tolerance, and you are quite right to say that there are many other factors at play here (besides the camera setting). Frankly, your problem statement did not precisely state the goal. Why are you now saying that copying images is pointless or illegal? You were the one to set up the problem. To 'reproduce the look' could mean anything from something vaguely reminiscent to an exact copy.

Very shallow DoF is not, contrary to the impression discussions of equivalence give, a widely used pictorial device;

I don't get your statement. It is a feature (or a bug, depending on your viewpoint) of almost all photography that most of everything is out of focus. Whether it is used intentionally as a pictorial device or not; it is simply a reality that we have to live with and work around.

Besides, shallow DOF, or rather, the amount of background blur, is a tool for isolating the subject. It is commonplace in e.g. wildlife photography, where quite often you get a rare opportunity at a certain subject, and there is lots of clutter that is not relevant to the image. You might not consider these examples (which are too numerous to list) important, but here we are again approaching deeply subjective territory.

I don't know whether you expect to settle such subjective and controversial matters, but I understand that you are pushing a certain taste or style of photography. I respect that, but I don't accept it as universal. Therefore I do not like the idea of subjugating technical/scientific concepts to (a particular) taste.

Do I really need to spell out an argument for why, if "equivalence" is not helpful in making photographs, it is not helpful in evaluating equipment for making photographs?

Well, if you want to. In my mind they are almost completely independent.
Why conflate two things if you can enjoy the freedom of thinking in two dimensions?

I like my kitchen knives to be very sharp. Sometimes it makes the food look and taste better, sometimes it is not noticeable.
Finding and perfecting ways to sharpen a knife is completely independent of perfecting culinary skills. I don't decide whether I sharpened the knife well by tasting the final meal, since there are too many factors playing into that. Instead, I inspect and try the blade using one or several standard tests. Having the blade perform well on these tests gives me the confidence I need to have in my tools. I don't subjugate the sharpness test to a specific style of cuisine, because I don't want to be constrained by this choice, and even when a sharp blade is not critical for the success of a meal, it makes the preparation more enjoyable and safer.