I suggest a simple answer: doubling linear resolution (lp/mm) and viewing big enough/close enough to see all the extra available detail gives half the DOF at the same f-stop compared to viewing of the lower resolution image, which means the same DOF as with half the f-stop with the lower resolution image.
P. S. One consequence is that as one attains higher and higher resolution, proportionately higher f-stops will typically be needed to adequately control OOF blur of subject elements for which one seeks a sharp image.
This is certainly a logical conclusion and one that occurred to me a while back when you, Howard and I were discussing DoF and comparing 35mm f8 with 8x10 f64. These two f/stops are supposed to be quivalent for DoF purposes. However, if you consider that no 35mm lenses are truly diffraction limited at f8 and LF lenses are
diffraction limited at f64; plus the fact the f64 image at the plane of focus will not be significantly degraded by the film but the f8 image will definitely be significantly degraded by the MTF loss of the film at those high resolutions, then one could easily come to the conclusion that for appropriate size prints, LF f/64 produces less
DoF than 35mm f/8.
On the other hand, if we were to ensure that both lenses (35mm and LF) were used at diffraction limited f/stops, which I think would mean using 35mm at f16 and 8x10 at f128, and if were to examine just the image from the lens, say projected on a screen, and ignore sensor and/or film limitations, then the images would have equivalent resolution at the plane of focus as well as equivalent DoF, wouldn't you agree?
I believe I've just rephrased the point you are making, for the sake of clarification (in my own mind, if no-one elses ).
So, if we start comparing lenses that are diffraction limited in one format with lenses at equivalent f/stops in another format, that are not diffraction limited,
we change the balance between maximum resolution and CoC and therefore change the appearance of DoF.
So how significant is this? Is it really an issue in practice? Is anyone going to worry about it?
I recall Jonathan commenting that large prints he made from 1Ds images, that had a fairly shallow DoF, appeared to have a similarly shallow DoF at small 5x7" sizes. Perhaps not quite as shallow, but no dramatic difference. Possibly different sharpening routines had skewed the results. Perhaps the way our eyes pick up detail is not proportional to the distance from the print. Perhaps the 'contrast' of the detail in the print skews the results. Perhaps the degree of OOF changes the degree to which our perception of DoF changes with changing distance from the print.
For example, most shots I've seen taken with the Canon 50/f1 have such an OOF background that the background is unrecognisable. If it's unrecognisable on a small print at close viewing distance, it's certainly going to be unrecognisable on a large print at the same viewing distance, but actually may be 'more' recognisable on the large print from a greater viewing distance. In such situations, the large high resolution print, which might appear to have less
DoF than the small, lower resolution print from close up, might appear to have greater
DoF than the smaller print, when viewed from a distance.
Perhaps an analogy here is the extremely low resolution jpeg image of say a face on your monitor. From close up, the features are not recognisable. Stand back a few metres and they are.
Are you now going to volunteer to do some practical tests to support your theory, BJL .