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From Wikipedia, the free encyclopedia

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.

A teleconverter attached between a camera and its lens
An object in the image plane appears smaller by the reciprocal of the teleconverter's rating when looked through it.
An Olympus DSLR with a 2× Olympus EC-20 teleconverter.[1]
1 – Camera lens
2 – Teleconverter
3 – Camera body

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.

A closely related device reduces the focal length. It is generally marketed under the name speed booster, though the term telecompressor has also been used. It is used in particular to adapt DSLR lenses to mirrorless cameras. Here the magnification is less than 1, typically 0.71× or 0.64×. In this case, the effective aperture increases accordingly.