What is "teleconverter"

n. (context photography English) A secondary lens mounted between the camera and a photographic lens so as to enlarge the central part of an image obtained by the objective lens.

A teleconverter (sometimes called tele extender) is a secondary lens which is mounted between the camera and a photographic lens. Its job is to enlarge the central part of an image obtained by the objective lens. For example, a 2× teleconverter for a 35 mm camera enlarges the central 12×18 mm part of an image to the size of 24×36 mm in the standard 35 mm film format. Teleconverters are typically made in 1.4×, 1.7×, 2× and 3× models, of which 1.4× and 2× are most common. The use of a 2× teleconverter gives the effect of using a lens with twice the focal length. It also decreases the intensity of the light reaching the film by a factor of 4 (an equivalent of doubling the focal ratio) as well as the resolution (by a factor of 2).