Diffraction can limit the sharpness of a good 50mm lens stopped down to f/16 on a 35mm camera. One can see the effect of stopping down too much by observing the image formed from 35mm film in an enlarger. Fine grain starts to get slightly fuzzy at about f/8. A big enlargement printed at f/16 will likely appear less sharp than at f/8. Normal focal lengths on 8x10 were often stopped down to f/64 with little sharpness loss due to diffraction. The choice of optimum aperture depends on format size, subject matter and distance, required DOF, viewer's preference, subject movement, print size, and too many other factors to discuss at length here.
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