Quote Originally Posted by E. von Hoegh View Post
Sorry. Diffraction depends solely upon the stop, it has nothing to do with focal length. The diffraction limit in LP/MM can be approximated by dividing the stop # into 1500. At f:45 the diffraction limit is about 30 LP/MM regardless of the focal length of the lens.
The folks here have a nice write-up on this:

Pity they aren't clear between "aperture" and "aperture ratio", f/16 most definitely being the latter. But I see this mistake thrown around a lot; it's good digital cameras are removing the need to think from photography. That way the Art can shine through.