Wiki 是个好东西 “where f is the focal length, and D is the diameter of the entrance pupil ( effective aperture). It is customary to write f-numbers preceded by f/,[2] which forms a mathematical expression of the entrance pupil diameter in terms of f and N. For example, if a lens's focal length is 10 mm and its entrance pupil diameter is 5 mm, the f-number is 2 and the aperture size would be expressed as f/2”.