F-number

 

In optics, the f-number (sometimes called focal ratio, f-ratio, or relative aperture) of an optical system expresses the diameter of the entrance pupil in terms of the focal length of the lens; in simpler terms, the f-number is the focal length divided by the "effective" aperture diameter.

             F-number=f/D

    where f is the focal length, and D is the diameter of the

 entrance pupil.