FWIW, I did a printing session today and performed some more extensive tests on some of my 50mm lenses than I've done before, focusing on performance at a variety of apertures. I tested each lens at four apertures (wide open [f/3.5 or f/2.8], f/5.6, f/8, and minimum aperture [f/11 or f/16]) in both the center and the periphery. I tested a 6-element Nikon EL-Nikkor f/2.8, a 5-element Vega-11U f/2.8, a 4-element Durst Neotaron f/2.8, and a 4-element Industar-96U f/3.5. I used a negative I shot for this purpose last year; it's got text samples at various sizes in both the center and the periphery, shot on Ilford Pan F+ film, which makes it easy to judge sharpness. I found that the optimum aperture for the center exposure was f/5.6 or f/8 for all of the lenses I tested (most leaned towards f/5.6, but some were virtually indistinguishable between their f/5.6 and f/8 exposures). Note that I didn't test the f/4 aperture that MichaelBriggs mentioned. In the periphery, it was another story; most got clearer up to the minimum aperture. (Of course, the true optimum could be somewhere in-between f/8 and the minimum.) The Nikon's f/8 and f/16 results were indistinguishable, though, and the difference between f/8 and f/11 was pretty small for the Vega-11U. The periphery was also where the differences were most pronounced, both in terms of differences over the aperture range and lens-to-lens differences. Overall, for my lenses I think using f/8 is the best compromise between center and edge sharpness if making uncropped enlargements in which edge-to-edge sharpness is important. When cropping or when subjects in the center or periphery are more important, this might change. I did this test with B&W film and paper; I have no idea how color might complicate things.