Originally posted by Recercare When looking at Yoshihiko Takinami's resolution tests it seems like many K/M/A lenses have a sweetspot between F11 and F16. This contradicts my experience with manual focus lenses. I usually get the sharpest results between F5.6 and F8.
I think the Takinami resolution tests are of limited use, as light piping in the film will have compromised the resolution measurements. The only way to measure resolution of a lens is using a microscope at the back of it, measuring the aerial resolution, removing the degredation of the sensor/film. I've tried measuring resolution from test charts using film and it was much harder than just using my DSLR, but the DSLR is limited to about 100lp/mm whereas many prime lenses will go higher than that.
There's an interesting review on Ken Rockwell's site of various 50mm Nikon lenses and a Zeiss thrown into the mix. His conclusion is that there is more variation in a lens according to the aperture being shot at, than amongst lenses. Its worth a read:
Nikon 50mm Lens Comparison
Stopping down a tad means you are using a smaller area of the elements, so that chromatic aberations come under control. Conversely, diffraction kicks in at smaller apertures.