When I was about 13, having exhausted the limits of my Kodak Pocket Junior camera (a 120 roll-film, fixed-focus folder with a single-element lens and two shutter speeds), I pestered my father for a "real" camera until he gave in. I left the camera store the proud owner of a 2 1/4 x 2 1/4 Korelle Reflex I SLR with speeds from 1/25 to 1/500 sec, an interchangeable 75mm f/3.5 Ludwig-Dresden Victar lens, and a Weston 650 lightmeter.
My father explained that the further I closed down the aperture, the sharper the picture would be. At f/3.5, the lens was barely sharp enough at the image center; edges and corners were less so. Stopping the lens down to about f/11 provided adequate central sharpness, and it improved the corners somewhat.