Telecompressor
A telecompressor or focal reducer is an optical element used to reduce focal length, increase lens speed, and in some instances improve MTF 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.
Lens adapters that include telecompressors have recently have become very popular with people using digital micro four thirds cameras. By combining a telecompressor within a lens adapter, micro four thirds 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]
Telecompressors were used in early digital SLR systems like the Minolta RD-175 and the Nikon E series. The technology of the time necessitated relatively small sensor sizes and limited digital ISOs. 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]