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.
It is a dimensionless number that is a quantitative measure of lens speed, and an important concept in photography. It is also known as the focal ratio, f-ratio, f-stop, or relative aperture.
The f-number is commonly indicated using a hooked f with the format f/N, where N is the f-number.
Formula:
where,
f = focal length
D = diameter of the entrance pupil
N = f Number
f = focal length
D = diameter of the entrance pupil
N = f Number