Hi, Asher,
Another take away is that one should consider both f stops and pixel density in doing work for some particular end result.
Getting increase in DOF with smaller aperture costs loss of resolving power at a contrast level that is increasingly degrading to the image quality.
I personally hardly ever have an aperture beyond 5.6 with a APS-C sized or full sizes "35" mm sensor.
With LF, it jumps to f16.
Sure.
Of course, there is no single metric that expresses the "resolution" of a system, but we can certainly adopt some consistent arbitrary metric that will allow us to compare the "resolution" performance of two postulated systems. We might for example use the spatial frequency at the focal plane, in
cycles per picture height where the MTF drops to 20% of its "low frequency" value.
Now let's only consider the impact of diffraction on resolution.
If we do that, then if for a format with a diagonal size of about 43 mm (ff35), we get a certain resultion (on the arbitrary basis adopted above) with an aperture of f/5.6, then with a format with a diagonal size of 55 mm, we would get that same resolution with an aperture of f/7.2.
To get that same resolution performance at an aperture of f/16, we would need to contemplate a format with a diagonal dimension of 123 mm (e.g., 74 mm × 98 mm). (I am ignoring the slight complication of differences in aspect ratio.)
Your discussion was quite reasonable couched in terms of the desire to use a "small enough" aperture to attain certain depth of field performance. Of course, here again we must be careful as to the conditions of comparison.
A pivotal matter here is our criterion for "negligible" blurring (as codified by our choice of a circle of confusion diameter limit). Two common outlooks are often used there.
A. We adopt a criterion of based on human visual acuity, assuming viewing of the image under certain highly-arbitrary conditions. That is, we consider as negligible that amount of blurring from imperfect focus that could not be noticed by the viewer (assuming that this blurring is not "swamped" by the blurring from diffraction and lens aberrations).
B. We adopt a criterion based on the resultion of the optical system, considering as negligible that amount of blurring by imperfect focus that would not significantly degrade the system resolution at the focal plane.
If we choose to work with criterion B, we have no hope of finding any general rule to help us see how diffraction can influence our ability to get a certain DoF performance with different format sizes.
So I will adopt criterion A as the premise for adopting a COCDL for DoF reckoning.
So the overall set of conditions I will adopt for comparison of DoF performance will be:
1. Focal length to give a consistent field of view.
2. Same focus distance
3. Same COCDL expressed as a fraction of the image dimensions.
4. Same f-number (except that in this exercise we will change that as needed for our purposes).
Now we will consider:
• camera X (with a format size of 30 mm × 40 mm, not a recognized format, but handy numbers)
• camera Y (with a format size of 60 mm × 80 mm, twice as large).
We will adopt a COCDL of 1/1400 the frame diagonal dimension in both cases.
We will assume focus at a distance of 100 m in both cases.
We will start by using, on camera X, a focal length of 100 mm and an aperture of f/2.0. The total depth of field is reckoned as about 300 m.
Now we will switch to camera Y, using a focal length of 200 mm, and for the moment still use an aperture of f/5.6. The total depth of field is now reckoned as about 81 m.
To regain the total depth of field of the previous case, we would need to adopt an aperture of about f/4.
This, simplistically,
under the conditions of comparison I had adopted, we find that, to maintain the same total depth of field as we increase the format size, we must increase the f-number roughly proportionately to the format size (and of course this is a well known relationship).
But we saw above that, in order to have a consistent impact of diffraction on image resolution (normalized to image size; that is, as would be perceived by a viewer observing a consistent size print at a consistent distance), we may use an f-number that is proportional to format size.
Thus I don't see how an increase in format size helps us to attain a certain DoF objective in the situation where diffraction limits our ability to use an arbitrarily-small aperture.
This is of course all based on about 15 minutes of thought (and before breakfast as well). I may have gone badly wrong someplace.
Best regards,
Doug