Been following this discussion with interest.
Tom: though I follow what you're saying, that you're trying to describe aperture in absolute terms rather than a ratio (f-number) which would vary the actual size of the aperture diaphragm depending on the focal length of the lens. This does logically makes sense since we're talking about squeezing photons through a small hole and the absolute size of that hole should matter, however at least according to this:
http://www.cambridgeincolour.com/tut...hotography.htm
scroll to the technical notes at the bottom, the larger actual aperture opening of a telephoto lens is offset by the greater distance the light must travel in the longer focal length lens so it seems it's appropriate to use the ratio aperture approach and not actual aperture size as it wouldn't matter.
Unless I missed it I'm also wondering why no one mentioned pixel pitch vs airy disc on this discussion. It seems fundamental when talking about diffraction.
AK: I wonder how much of what u mentioned about lens design and performance at different apertures is just that - lens characteristics, and not diffraction related. At least in theory it seems diffraction will affect a lens designed to perform best at wide apertures as well as one designed to perform best stopped down equally. But what diffraction robs will be relative to that lens and the normal gains with dof as u stop down and IMO cannot be compared to another lens of the same focal length.