21mm

21 millimeters (21 mm) is a unit of length in the metric system equal to 0.021 meters or 2.1 centimeters. It is used to specify dimensions such as diameters, lengths, or spacings in engineering, manufacturing, and everyday measurement.

In photography and optics, 21 mm commonly refers to a wide-angle focal length for camera lenses. On a full-frame sensor, a 21 mm lens provides a broad field of view and is often used for landscapes, architecture, and interiors. Such lenses can exaggerate perspective and reveal more of a scene, but edges may show distortion depending on the lens design. On cameras with smaller sensors, the effective field of view changes with the crop factor: roughly equivalent to about 32 mm on APS-C bodies (with a 1.5x crop) and about 42 mm on Micro Four Thirds bodies (with a 2x crop). The choice of aperture and whether the lens is rectilinear or fisheye also affects distortion and edge performance.

In practice, 21 mm lenses balance a wide perspective with relatively manageable distortion compared to shorter wide angles, making them popular for architectural and interior work where some wide context is desired without overly distorted edges. The exact look depends on sensor size, lens construction, and shooting distance.

See also: millimeter, focal length, wide-angle lens, crop factor.