Telecompressor

A telecompressor or focal reducer is an optical element used to reduce focal length, increase lens speed, and in some instances improve optical transfer function (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. A combined system of a lens and a 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 become popular with people using digital mirrorless cameras. By combining a telecompressor within a lens adapter, mirrorless cameras can use the lenses of both digital single-lens reflex cameras (DSLRs) 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 and KIPON.

For a refractor telescope or simple camera lens, the new effective focal length f_n is given by:

where f_o = original focal length of telescope, d = distance from telecompressor to image plane, and f_r = focal length of telecompressor.

For a reflecting telescope, the calculation is the same. However, since the telecompressor increases the field of view, there could be vignetting in the image, depending on the sizes of the secondary mirror and the telescope tube.

...
Wikipedia