ˈ ̷ ̷ˌ ̷ ̷ ̷ ̷ noun
Etymology: f, symbol for focal length
: the ratio of the focal length to the entrance pupil diameter in an optical system or the objective of such a system ; specifically : a number following the symbol f/ that expresses the relative aperture of a camera lens, the smaller the number the brighter the image and therefore the shorter the exposure required
the f-number of a lens with an f/8 relative aperture is 8