lens speed

Definition: Measure of how little light is needed by a lens in order to work: a fast lens needs little light, a slow one needs more. * In practice this is measured by maximum aperture: large maximum aperture for a given class of lens equates with a fast lens, small maximum aperture with a slow lens. * E.g. f/2.8 is very fast for a 400mm lens, but for a 50mm, the aperture should be f/2 or lower for the lens to be considered fast.

Related Terms: f/number, relative aperture

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