Archived posting to the Leica Users Group, 1998/05/02
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Actually an F4.0 280mm lens is already stopped down to F4.0. It is the numerical aperture that is important in determining resolution. The approximate relationship between f number and numerical aperture is: f number = 1/2 x numerical aperture. The theoretical resolution of an f2.0 lens is 884 lines/mm, that of an f4.0 lens is 451 lines/mm, that of an f16 lens is 113 lines/mm, that of an f22 lens is 83 lines/mm. Diffraction limited means that the lens is capable of resolving the number of lines/mm indicated by its f number on optical test bench. In reality, this limit is never approached when recording actual images on film. - LarryZ