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.
And the F stop is a fraction of the focal length. f16 on a 500mm lens is a whole lot larger than f16 on a 50mm lens. It's the relation of the size of the hole to the wavelength of light going through it that causes diffraction.