Photo Dictionary f-number - (also called f-stop) a number that is used to designate the relative size of the aperture in the lens. The usual sequence of f-numbers is: f1, f1.4, f2, f2.8, f4, f5.6, f8, f11, f16, f22, f32, f45, f64, etc. From Photography by Barbara Upton and John Upton © 1981 Second Edition page … Continue reading f-number