DOF and Cropping take 2

Started Feb 11, 2014 | Discussions thread
Great Bustard Forum Pro • Posts: 40,688
"Dangerous"? Heh!

James O'Neill wrote:

etc etc

Joseph James is dangerous, because he talks about aperture diameters which people who aren't on their guard will read as "f/ number" it isn't : it is diameter of the hole in mm.

So Joseph James (me) is dangerous because people will not read/understand what he *clearly* spells out. For example, from the link above:

  • ...and aperture diameter (25mm / 1.4 = 31mm / 1.8 = 33mm / 1.9 = 50mm / 2.8 = 18mm).

Huh. Were it me, I'd say what's "dangerous" is your assumption that "aperture" must necessarily mean "f-ratio", as opposed to understanding what the three apertures are, and their significance:

Understanding the fundamental concepts of Equivalence requires making important distinctions between various terms which people often take to mean the same thing. It is very much akin to making the distinction between "mass" and "weight", two terms which most people take to mean the same thing, when, in fact, they measure two different (but related) quantities. While there are circumstances where making the distinction is unnecessary, there are other times when it is critical.

The first of these distinctions that needs to be made is between aperture and f-ratio. The term "aperture", by itself, is vague -- we need a qualifying adjective to be clear. There are three different terms using "aperture":

  1. The physical aperture (iris) is the smallest opening within a lens.
  2. The virtual aperture (entrance pupil) is the image of the physical aperture when looking through the FE (front element).
  3. The relative aperture (f-ratio) is the quotient of the focal length and the virtual aperture.

Thus, the "f" in an f-ratio stands for focal length. For example f/2 on a 50mm lens means the diameter of the virtual aperture (entrance pupil) is 50mm / 2 = 25mm. Likewise, a 50mm lens with a 25mm virtual aperture has an f-ratio of 50mm / 25mm = 2.

He talks about "total light" which isn't illumination in lux or lux seconds but lux-seconds * area. And so on. He's not actually wrong, but people will argue about two lenses with "the same aperture" using him for support and both will understand different things by "aperture". When we find ourselves at cross purposes with terms like "light" and "Aperture" we're in trouble !

The people who are "in trouble" are not merely the people who missed it the first time around, but the people who deny the truth when it is spelled out to them.

The idea of equivalence is problematic, because you CAN say. If I put my APS-C camera on a tripod with a 50mm lens set to f/8, and expose for 1/4 second with camera set to ISO 100 that will give me the same image as a 35mm film camera loaded with ISO 100 film with a 75mm lens set to f/12 exposed for 1 second.

It will not. The 35mm photo in your example will have been made from four times as much light as well as greater motion blur / camera shake if there is motion in the scene and/or a tripod or lens with IS is not used.

But the differences between film and digital rendering mean they aren't the same even if they are equivalent. By the same token when I compare two digital formats the sensor in the cameras are not identical in enough of their parameters to get the same DR, the Same noise, the same detail so it's massively unlikely that when if we can image equivalence we get the same results in the output.

So while no two photos from two different systems will ever be equal, Equivalent photos from different systems will be as similar as photos from different systems will get. Clearly, however, the point of choosing one system over another is not simply to get photos as close as possible to other systems (equivalent photos), but to get photos that look "better" (in each photographer's opinion) to what other systems can deliver (non-equivalent photos), or for the differences in operation (AF speed/accuracy, size, weight, frame rate, build, price, etc.).

We can compare systems in many different ways. The five parameters of Equivalence (same perspective, framing, DOF, shutter speed, and display size) are simply guidelines to comparing systems on the basis of the most similar visual properties of the final photo, and are certainly not a mandate that systems must be compared in such a fashion. Therefore, it is important to specify the purpose of the comparison, and then not artificially handicap one or the other system with the conditions of the comparison. Of course, this is not to say that there are not most certainly instances where a photographer is limited due to size, weight, and/or finances and would therefore compare systems within those constraints. However, it is important to interpret the results of the comparison in the context of the circumstances where the conditions of the comparison are valid.

The point of photography is making photos. As such, one doesn't choose the particular system to get images which are equivalent to another system. A person chooses a particular system for the best balance of the factors that matter to the them, such as price, size, weight, IQ, DOF range, available lenses, and/or operation. By understanding which settings on which system create equivalent images, the difference in their capabilities is more easily understood.

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