fstop

The f-stop, or f-number, is a dimensionless quantity that characterizes the size of a camera lens aperture relative to the focal length. It is denoted as f/N, where f is the focal length and N is the f-number. The effective aperture diameter is f/N, and the light-gathering area scales with 1/N^2. The same N across a lens yields similar exposure when shutter speed and ISO are constant.

In photography, each full stop change doubles or halves the amount of light reaching the sensor. Common steps include f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22. A lower f-number lets in more light; a higher f-number reduces it.

Aperture affects depth of field: larger openings (low f-numbers) give shallower depth of field; small openings (high f-numbers) increase depth of field. At very small apertures, diffraction can soften image sharpness and counteract the gains in DOF.

Some light loss occurs in lenses due to transmission. The T-stop is a related measure that accounts for actual transmitted light through the lens. In cinematography, T-stops are used for precise exposure; in still photography, f-stops are the standard convention.

The f-stop interacts with shutter speed and ISO as part of the exposure triangle. Lens design also matters: a maximum aperture corresponding to a low f-number yields a “fast” lens, beneficial in low light and for achieving shallow depth of field.

In summary, the f-stop is a core concept that governs exposure and depth of field, encoded on most lenses as f/ values, and it informs decisions about brightness, focus, and image sharpness.