Basically those two things are diffraction, and aberrations. They are opposing because closing a lens down increases diffraction, and openning it up increases aberrations. A lens is said to be diffraction limited when all the aberrations are corrected at the widest apperture. That is fairly easy to do with something like the old Kodak Ektar 200mm/f7.7 (covers 4x5, well actually 5x7 but it is not diffraction limited on 5x7) because the maximum aperture is only about f8. The French made Kinoptik 100mm/f2.0 (covers 35mm)which is also supposed to be diffraction limited is a very expensive lens as it is much harder to do with a fast lens. It is also harder to do with wider fields of view which is why the Ektar is not diffraction limited on 5x7.
The wider the aperture the higher the resolution, as long as the aberrations are fully corrected. The Kinoptik with its 50mm aperture has an aerial resolution of about 400 lines per millimeter, the Ektar with 1/2 that has about 200 lpm.
Now it is easier to make a fully corrected large aperture lens at shorter focal lengths, so that is why they claim shorter lenses have higher resolution. But that unless you are willing to pay astronomical amounts for diffraction limited lenses is only theoretically, especially today when they can make large lenses to the same tolerances as smaller ones.
Note: Those high resolution numbers I mentioned are for aerial images, examined with a microscope (also they are an approximation as I am too lazy to look up or calculate the actual figures). Overall resolution on film or sensor will alway be lower than the lowest resolution in the system which is usually the film or sensor resolution. As an example, with that Kinoptik (400 lpm), on Tech Pan (200 lpm) you would get something like 150 lpm overall, which is about the best your are going to do with what I know is available out there.
Also note that by the time you make your print that near impossible 150 lpm on 35mm is only equal to an easy to get 37.5 lpm on 4x5 which should put the whole thing into perspective.
-- graywolf http://graywolfphoto.com
"You might as well accept people as they are, you are not going to be able to change them anyway."
