Prime lens
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In film and photography, a prime lens is a photographic lens whose focal length is fixed, as opposed to a zoom lens, which has a variable focal length. Prime lenses are not as versatile as their zoom counterparts, but are often of superior optical quality and lower cost. This is because the designs are simpler (fewer moving parts), and can be optimized for one particular focal length, whereas zoom lenses must function at a variety of focal lengths.
Prime lenses usually have a larger maximum aperture (smaller f-number) than zoom lenses. This allows for photography in lower light (with the same shutter speed) as well as achieving a shallower depth of field.
In 35mm photography a "normal" prime commonly refers to a prime lens with 50mm focal length because it renders an image most like what is seen by the human eye. Digital cameras, due to their crop factor, will usually require shorter focal lengths to have the same magnification. Perspective is determined by the relative position of the camera and subject, not the focal length of the lens.
[edit] Alternative meaning of the term
In the early motion picture days the term prime was referring to the primary lens attached to the camera, as opposed to converter lenses, and without regard to whether it is fixed or variable focal length. Lens manufacturers such as Zeiss, Schneider, Century and others still make cine and video lenses catalogued as variable primes.
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