Tom wisely wrote:
"Actually it is both. Diffraction is based upon the diameter of the aperture diameter and the dispersion angle of the light rays which is affected by focal length. So the shorter the lens, and the smaller the aperture, the worse the diffraction.
"The lens will be best at the widest f-stop that is fully corrected. If that is also the maximum f-stop on the lens the lens is said to be diffraction limited (DL). There were actually quite a few DL lenses with maximum f-stops of f8.0 or so. I know of only one that was supposed to be DL at f2.0, the French made 100mm/2 Kinoptik (I suppose there may have been others that I have never heard of)."
Interesting. Pentax thinks that the DA 16-45 gives its best mtf performance wide open (f4.0) between 16 and 24 mm., and f4.5 thereafter. So is the DA 16-45 a diffraction limited lens? And in terms of everyday shooting, what does that mean? (I've used it at f16 when I wanted DOF, and that the results were stunning.)
FWIW, on my 1p bodies I used to shoot the FA 100 f2.8 macro at no more than f11. On the *ist D I started pushing the f-stop, first to 16, then to 22. Even at f22 the results are very, very sharp, and I will continue to use that aperture when I can.
Joe
