F-number
RATIO OF THE FOCAL LENGTH TO THE DIAMETER OF THE ENTRANCE PUPIL
F/stop; F stop; F number; F-stop; Focal ratio; F-Stop; F-Number; Relative aperture; F stops; T stop; Fstop; Relative Aperture; Working f-number; Tstop; Tstops; T-stops; T-stop; T stops; F-stops; F/number; Half stop; Stop number; T-stop system; F Number; F/; Ƒ-number; Transmission stop; T-number (photography); T number (photography); H stop; H-stops; H-stop; H stops; H number (photography); H-number (photography); F/11
In optics, the f-number of an optical system such as a camera lens is the ratio of the system's focal length to the diameter of the entrance pupil ("clear aperture").Smith, Warren Modern Optical Engineering, 4th Ed.