Actually, DOF, by definition, is this:Code: [Select]
DoF = (2 * N * c * f^2 * s^2) / (f^4 - (N^2 * c^2 * s^2))
The factor for CoC, circle of confusion, is c. It is effectively arbitrary. [...]
I suggest you read this first:
Do not miss this paragraph:
If the original image is enlarged to make the final image, the circle of confusion in the original image must be smaller than that in the final image by the ratio of enlargement. Cropping an image and enlarging to the same size final image as an uncropped image taken under the same conditions is equivalent to using a smaller format under the same conditions, so the cropped image has less DOF. (Stroebel 1976, 134, 136–37).
Note that the acceptable circle of confusion values for these formats are different because of the relative amount of magnification each format will need in order to be projected on a full-sized movie screen.
"to be projected on a full-sized movie screen"
That would be an enlargement by a factor of 20x, 30x, maybe more? Of course CoC is going to matter with such an enlargement, despite the fact that you sit back by 30 to 50 feet. I don't think most photographers enlarge much more than 2x, and the vast majority of photographers reduce their images (usually quite considerably) for publication online.
As for the quote from Strobel, in the 1970s, film was the primary means by which photography was done. CoC was usually considerably larger back then than it is today with pixels less than 10 microns in size. Particularly in the case of large format cameras. With such large CoC sizes, it was a more important factor, even for something as relatively simple as a 2-3x enlargement. More recent films manufactured with more modern technology have produced silver halide film grains on the order of a few microns in size (one film in particular that was used by Zeiss to test high grade fast optics apparently was capable of resolving 400lp/mm, more than any sensor that I know of as of yet), but generally speaking CoC sizes today are quite small when compared to the film of the 1970s (a CoC that is 2x the pixel pitch of the average pixel size today, which is around 5µm, would be 0.01mm...a CoC for medium and large format film from the '70s would be on the order of 0.2-0.3mm...a difference by a factor of over 20x). It takes a pretty significant enlargement (say projecting on a large movie screen) to make CoC a meaningful aspect of DoF for the majority of photographers today.
I would point out that I speak from experience. I print at 24x36, 30x40, and 32x48 on a fairly frequent basis. For those particular prints hanging on my walls, they are usually viewed standing back ten feet or so...no one has ever complained about my depth of field being too thin or too large, or that the inaccuracies in my 7D AF resulted in a horribly misfocused subject totally unworthy of such a large honor.
Anyway...points have been made. The debate, once again, is going nowhere. I'm going to bed. It's up to you and Neuro now.