Can anyone tell me the smallest aperture one would expect to give optimum results for macro shots with a 100 mm? I recently got a Canon EF 100 f2.8 macro (used on an XTi) and all of the shots from f2.8 to 11 were nice and sharp, but I also took a bunch at f25 and they are a little soft. The f25 shots were mostly taken very close up, but one taken at infinity is sharper than the others - would diffraction be worse at closer distances? I had a 105 macro for my film camera and seem to remember using f16 and f22 without any problem, although I wasn't blowing the images up on a screen the way I am now. I read somewhere that diffraction effects somehow are worse in digital, or perhaps just more noticeable?