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Depends on sensor size. For a FF body, I would say f11 (sharp and still minimal diffraction). However, the overall picture may appear sharper at f16 (or even higher) due to DOF. On a crop body, max sharpness probably occurs around f8, but f11 would give better DOF and diffraction isn't too bad yet. If you are shooting landscape and are looking for max sharpness (I would assume on a tripod) then just take a picture at f8, f11, f16, f22 and decide which is sharpest for the actual shooting situation. Digital film is cheap.