Prime lens

In film and photography, a prime lens is either a photographic lens whose focal length is fixed, as opposed to a zoom lens, or it is the primary lens in a combination lens system.

Confusion can sometimes result due to the two meanings of the term if the context does not make the interpretation clear. Alternative terms primary focal length, fixed focal length, and FFL are sometimes used to avoid ambiguity.

The term prime has come to be used as the opposite of zoom; that is, a prime lens is a fixed-focal-length, or unifocal lens, while a zoom lens has a variable focal length.

While a prime lens of a given focal length is less versatile than a zoom, it is often of superior optical quality, wider maximum aperture, lighter weight, smaller size. These advantages stem from having fewer moving parts, optical elements optimized for one particular focal length, and a less complicated lens formula which means that they suffer from fewer problems of optical aberration. The larger maximum aperture (smaller f-number) allows photography in lower light, and a shallower depth of field.

A normal lens or "normal prime" is a lens with a focal length about equal to the diagonal size of the film or sensor format, or that reproduces perspective that generally looks "natural" to a human observer under normal viewing conditions.

An alternative and apparently somewhat older meaning of the term prime lens is the main lens in a combination lens system. When the camera lens is used with some other optical device, such as a close-up lens, teleconverter, or teleside converter, the camera lens itself is properly called the prime lens. Prime is here used in the sense of primary, chief, original, first in order, etc.

Lens manufacturers such as ARRI Media, ISCO Precision Optics,Schneider,Carl Zeiss AG,Canon and others still make variable focal length cine and video lenses regularly catalogued as variable prime lenses. A variable prime is sometimes distinguished from a "true zoom" in that the latter maintains focus as the focal length is varied.

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