Originally posted by sjwoo So for this lens, middle of the f-stop would be around f8. What is it about that midrange that makes for the sharpest photographs?
Search on DIFFRACTION LIMIT. But here's an overview: basically, when light hits the edge of an opening, it gets deflected, diffracted. As you know, aperture is measured in f-stops, which are the RATIO of the focal length to the size of the aperture opening. So f/1, the aperture and focal length are the same. At f/2, the aperture diameter is half the size of the focal length. Et cetera.
If the focal length is long, even a small f-stop is still a rather large hole. But on a short lens, that hole is pretty small. So, when a short lens is stopped down, a noticeable portion of the light passing through the aperture has bounced off its edges. It's deflected, diffracted; it's no longer going where it should on the film or sensor, so the image is a bit fuzzy.
Frame size has a major role here too, as your DIFFRACTION LIMIT research will show. Ansel Adams and crew worked with 8x10 inch (20x25cm) view cameras, with apertures down to f/64. But they also used 320mm lenses, where f/64 is a 5mm hole. With a 50mm lens, f/64 is only 0.78mm across. The combination of small frame and small hole makes for increased diffraction.
But is that important? If you're shooting ultra-macros or detailed landscapes that will be greatly enlarged and closely inspected, then yes. If you're shooting snapshots, grab shots, street scenes, life around you, and you're not intent on 30m blowups, then no. But try it yourself: Put camera on tripod, set to Av mode. Aim at something nice. (Don't tell us her name.) Shoot at entire range of f-stops. (Depending on the lens, you may need to bracket the shutter speeds.) Now view the pictures at a normal distance, looking for objectionable fuzziness. Unless you're pixel-peeping, you may have a hard time finding any.
Why is midrange considered best? Because you avoid the softness of wide-open, and the diffraction of closed-down. But lensmakers set their smallest apertures so that, with normal usage, diffraction ISN'T a problem -- unless you're doing extreme blowups that will be closely inspected. That 30m blowup probably won't be gone over with a magnifying glass. So don't sweat it.