I did a bit more reading on diffraction, and I was only sort of correct in what I said before about APS-C and f/16. Turns out that's just a general rule and it depends on the lens. Consumer quality lenses that perform best when stopped down will actually perform better at very small apertures than will high end lenses that are very good wide-open. So a cheaper lens that is best at f/8 might not start seeing diffraction effects until f/16, but a pro lens that is best at f/4 might only go to f/8 before sharpness starts to drop.
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