focal length
The height and width of the scene captured by a camera lens, which is known as the "field of view." In the past, lenses with fixed focal lengths were commonly used, which means every picture was captured with the same field of view. Today, most lenses are "zoom lenses" with a variable range of focal lengths and a variable field of view. See f-stop. |
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| These two pictures were taken at both ends of a Nikon 18-70 mm zoom lens on a digital SLR. At 18 mm (top), the field of view is the widest. At 70 mm (bottom), the scene is the narrowest. The 18-70 mm on the Nikon camera is equivalent to a 27-105 mm lens on a 35 mm analog SLR camera (see crop factor). |
focal length, distance
the distance from the focal point of a lens or mirror to the reflecting surface of the mirror or the centre point of the lens
focal length [′fō·kəl ‚leŋkth]
(optics)
The distance from the focal point of a lens or curved mirror to the principal point; for a thin lens it is approximately the distance from the focal point to the lens. Also known as focal distance.
Focal length
A measure of the collecting or diverging power of a lens or an optical system. Focal length, usually designated f ′ in formulas, is measured by the distance of the focal point (the point where the image of a parallel entering bundle of light rays is formed) from the lens, or more exactly by the distance from the principal point to the focal point. See Geometrical optics
The power of a lens system is equal to n′/f ′, where n′ is the refractive index in the image space (n′ is usually equal to unity). A lens of zero power is said to be afocal. Telescopes are afocal lens systems. See Diopter, Lens (optics), Telescope
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