A prime lens is a mechanical assembly of lenses 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 photographing in lower light as well as achieving a narrower depth of field.