Secondary lens

In photography, a secondary lens is a lens designed to be used in conjunction with another lens, called the primary lens.

A secondary lens may be designed to be used either in front of the primary lens, between it and the subject, or behind the primary lens, between it and the film.

Secondary lenses are typically simpler in mechanical design than other photographic lenses. Normally but not always there is no provision for control of focus or aperture, as these are achieved by the controls on the primary lens.

Examples of secondary lenses designed to be used in front of the primary lens are:

All of these typically mount on the filter ring of the primary lens.

Front-mounting secondary lenses are commonly used to provide wide-angle and telephoto lenses for cameras which lack the facility to change the primary lens, such as consumer and "prosumer" video cameras and camcorders, bridge cameras, point-and-shoot cameras and some older consumer medium format cameras.

The most common secondary lens used behind the primary lens is the teleconverter. A typical teleconverter has a male and a female lens mount of the same type, to allow it to be used between a primary lens and a camera body of the matching system. The resulting combination has a greater focal length than the primary lens alone, and the same absolute aperture, so its f-number is increased accordingly.

Some teleconverters are dedicated or matched to particular primary lenses, normally one or more lenses by the same manufacturer as the teleconvertor, and when used in this combination deliver quality comparable to the primary lens. Others are general-purpose, and in use typically offer less image quality then a primary lens of the same quality, but allow a camera kit to offer a wider variety of focal lengths with a smaller number of lenses, saving in both cost and weight.

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