A telecompressor or focal reducer is an optical element which amateur astronomers use to reduce the focal length of a telescope. Such devices are also available to photographers, for using lenses between systems with different sensor sizes.
Mainly used for astrophotography, the telecompressor gives a wider angle of view, a sharper picture, and reduces the exposure time needed. These are useful when taking pictures of nearby large objects, such as nebulae. The effects and uses of the telecompressor are largely opposite to those of the teleconverter or barlow lens.
One effect of a telecompressor that it reduces the diameter of the image circle. In practice this means that a lens meant for larger format can be used on a smaller sensor without crop factor. For example, a lens made for 135 format film (or "full frame") can be combined with a telecompressor to fill the image circle of a APS-C sized sensor. Without telecompressor the image on the APS-C sensor is cropped and some light is lost.
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