Originally posted by brkl Cheers, Ben. I'm sure I wouldn't have been that concise

Am I right in that diffraction has to do with the actual non-relative aperture? The aperture diaphragm opening of a 90mm lens at f64 is about 1.4mm, same as an 22mm lens at f16. The larger your format, the longer your focal lengths and so bigger non-relative apertures.

You are correct. If we have a short look at how resolution of lenses is defined (simplified):

RESOLVING POWER RP = 1/1.22 lambda A ; where lambda is wavelength of the light an A the relative aperture (aka aperture number) ; If we use the formulae for the angular resolution of a lens, which I personally find much more practical, we get RES = 1.22 lambda/Diameter ; and that is fully independend of the f-stop/relative aperture, as it simply does not take fl into account.

As you see, resolving power or resolution goes down, when A gets bigger, i.e.the lens diameter D gets smaller. Indeed, the resolving power is only dependend on the real diameter of the open aperture (given in mm or whatever) and independent of focal length and thus of the relative aperture (f-stops).

With photographic lenses, the things get more complicated, as the entrance pupil gets the defining factor for resolving power (and its limits due to diffraction). The entrance pupil is only the projected image of the real aperture, as seen from the front of the lens. The diameter of that entrance pupil is not only dependent on the real size of the aperture opening (indeed it may be very different), but also from focal length and the construction of the lens. So, I think, that it is near impossible to calculate, what real resolution a specific modern lens (things are easier with older, simpler and often highly symmetrical lens designs) may achieve, just by knowing the aperture number and the fl.

Ben