It's important to realize that the DOF's being a function of format size, is only an artifact of the way apertures are expressed in photography. The f/ratio is a complicated way to express aperture that is used in photography because it results in uniform exposure between all format sizes. The rest of the optics world (microscopy and photolithography are what I have experience with) uses Numerical Aperture to define aperture.
When defined in this more geometrically simple way, DOF-per-given-aperture is not a function of format size. However if cameras expressed aperture in Numerical Aperture, different apertures would not give the same exposure value across different format sizes. A given aperture number would be slower on larger cameras, but would give the same DOF across all format sizes. The F/ratio system is used because, it is felt, that it is more important that a given aperture number give consistent exposure across format sizes than it is that it give consistent DOF across format sizes.
In microscopy exposure isn't as important because you can just turn the light up a bit, but being able to calculate DOF for any possible magnification is very simplifying. Since exposure is important in photography, photography uses the F/ratio to express aperture, which takes into account the combined effect of aperture area and the inverse way that light falls off as the focal length increases. Using this system, f/8 gives the same exposure on 8x10 as on 35mm, but the DOF is much different between the formats.
Now, the way that DOF itself is quantified using certain criteria of unsharpness and so on, is another matter, but I hope that explains a bit about why DOF varies with format size in photography.
Last edited by BetterSense; 07-08-2009 at 11:37 PM. Click to view previous post history.
f/22 and be there.