Telecompressor
A telecompressor or focal reducer is an optical element used to reduce focal length, increase lens speed, and in some instances improve OTF performance. Popular applications include photography, videography, and astrophotography. In astrophotography, these qualities are most desirable 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. Combined system of a lens and focal reducer has smaller back focus than the lens alone; this places restrictions on lenses and cameras that focal reducer might be used with.
Lens adapters that include telecompressors have recently have become popular with people using digital mirrorless cameras. By combining a telecompressor within a lens adapter, mirrorless cameras can use the lenses of both DSLR (Digital single-lens reflex cameras) and film-based SLR (Single-lens reflex cameras), whilst 'magically' gaining a stop (F-number) i.e. giving the performance of a more expensive lens on a cheaper lens. The most popular of these products are known by the company that makes them: metabones[1]
Keplerian (relay) telecompressors
Telecompressors were used in early digital SLR systems like the Minolta RD-175 and the Nikon E series. The technology of the time used relatively small sensor sizes, so lenses designed for 135 film could not be used with their native field of view without additional optics used. Implementing a telecompressor helped to mitigate these limitations. One effect of a telecompressor is that it reduces the diameter of the image circle. This means that a lens meant for a larger format can be used on a smaller sensor with a reduced crop factor.[2]