Definition: Prime Lens

Definition: Prime Lens | Glossary entry

Prime Lens

The term Prime Lens has one of two meanings:

  1. A photographic lens which has a fixed focal length.
  2. The primary photographic lens in a combination set of lenses.
A Prime Lens

A prime lens has a characteristic fixed focal length (as opposed to a zoom lens which has a variable focal length). It also has a fixed angle of view.

In addition to the difference in the focal length a prime lens is unable to magnify the image like a zoom lens. Nevertheless prime lenses do have the ability to focus on an object. They are often of very high optical quality. The simple configuration of lenses in the body make it possible to avoid the aberrations which are associated with many glass elements. A simple optical configuration also leads to lighter weight, smaller size and often low cost (although the latter is dependent on lens specification).

Simple optics also leads to one of the main advantages of a prime lens, a large aperture. This enables the lens to allow more light through the lens when the iris-diaphragm is fully open. As a consequence the prime lens will be able work more effectively in low light situations. The large aperture also allows for a shallower depth of field affording a high degree of focus isolation in compositions.

Prime lenses are often fitted with more iris leaves which helps the lens create near circular aperture and hence circular bokeh. The latter is more aesthetically pleasing than shaped bokeh which results from fewer iris blades.

A wide range of prime lenses are made by lens manufacturers. Most popular is the so called, “nifty fifty”. This is the popular name for the 50mm focal length prime lens. It has been calculated that the lens that most closely approximates the way the human eye works is close to 50mm. Many people buy a 50mm lens so that they are able to compose photographs that are good approximations of the way we might see a scene. However, prime lenses are also made in a wide range of other focal lengths.

The Primary Lens

Traditionally the main camera lens was supplemented with add-on lens components types when doing specialist photographic work. For example extension lenses, close-up lenses, tele-converter lenses and others. These all optically modify the camera lens. The camera lens is therefore the ‘prime lens’ in a combination of lenses collected together to form a new optical result.

This is a somewhat confusing way to use the term ‘prime lens’. Modern camera lenses are not as often supplemented with add-on lenses as once was the case. The term prime lens to denote the main lens in an assembly of lenses is a less useful term than in the past. It is used more in specialist realms of photography recently.