Photo Term Series #6: f-stop, f/stop or f-number

“the f-number (sometimes called focal ratio, f-ratio, or relative aperture^[1]) of an optical system expresses the diameter of the entrance pupil in terms of the effective focal length of the lens. It is the quantitative measure of lens speed, an important concept in photography” – Wikipedia

In other words, the f-stop is the ratio of lens focal length to the effective aperture open diameter. Each f-stop step increment represents a doubling or halving of the lens aperture open area and thus the amount of light admitted into the lens

[wide open] 1.0, 1.4, 2.0, 2.8, 4.0, 5.6, 8, 11, 16, 23, 32 [closed down, pinhole].

Note: f-stop values are a key reference in discussing Depth of Field (DOF).
(See Photo Term Series Post #7: Depth of Field (DOF))

[tags]aperture, f-stop, f/stop, photography, term, photo, terminology, Depth of Field, DOF, [/tags]