Quote Originally Posted by Drew Bedo View Post
I understand that it is a ratio of focal length to aperture . . .but can't quite get the math right.
All said above on the math is just right.... But one more thing though: the ratio is not of focal length to aperture but of the focal legth to the entrance pupil diameter. In most of the 'normal' lenses, the entrance pupil is slightly larger than the iris opening. It is considerably larger than the iris in telephoto lenses, and it is much smaller in retrofocus lenses. That presents some difficulties in finding the actual apertures in an unknown lens.