teleconverter

thumb|A teleconverter attached between a camera and its lens thumb|An object in the image plane appears smaller by the reciprocal of the teleconverter's rating when looked through it. thumb|right|An Olympus DSLR with a 2× Olympus EC-20 teleconverter.1 – Camera lens 2 – Teleconverter 3 – Camera body A teleconverter (sometimes called tele extender) is a secondary lens mounted between a camera and a photographic lens which enlarges the central part of an image obtained by the 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× variants, with 1.4× and 2× being the most common. A 2× teleconverter doubles the apparent focal length of a given lens. Teleconverters also decrease the intensity of the light that reaches the film plane (or sensor) by the square of its magnification. A 2× teleconverter reduces the light to 1/4, doubles the focal ratio and halves the resolution of the master lens it is connected to. This, however, does not necessarily halve the resolution of the digital image.