FNumber

F-number, denoted N or f/#, is a dimensionless quantity used in optics to describe the ratio between the focal length f of a lens and the diameter D of the entrance pupil. It is defined as N = f/D. The entrance pupil is the image of the aperture stop as seen from the front of the lens, which may differ from the physical aperture due to lens design.

Because the image brightness on the sensor scales with the area of the aperture, and area ∝ D^2, the exposure is proportional to 1/N^2 for fixed shutter speed, ISO, and scene illumination. In photography, each standard f-number step increases N by roughly a factor of sqrt(2), reducing exposure by about one stop.

A lower N (larger aperture) produces brighter images and shallower depth of field, while a higher N yields deeper field and more diffraction. At very small apertures like f/16 or f/22, diffraction can soften sharpness, offsetting depth-of-field gains.

The f-number is used in photography and cinematography to control exposure and depth of field. It is a standard part of lens specifications, usually written as f/2.8, f/4, f/5.6, etc., and is sometimes referred to as an f-stop.