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A telecompressor or focal reducer is an optical element used to reduce
focal length The focal length of an optical system is a measure of how strongly the system converges or diverges light; it is the inverse of the system's optical power. A positive focal length indicates that a system converges light, while a negative foca ...
, increase lens speed, and in some instances improve optical transfer function (OTF) performance. It is also widely known under the name “Speed Booster”, which is the commercial name of a line of telecompressors by the manufacturer Metabones. Popular applications include
photography Photography is the art, application, and practice of creating durable images by recording light, either electronically by means of an image sensor, or chemically by means of a light-sensitive material such as photographic film. It is employed ...
, videography, and astrophotography. In astrophotography, these qualities are most desirable when taking pictures of nearby large objects, such as
nebula A nebula ('cloud' or 'fog' in Latin; pl. nebulae, nebulæ or nebulas) is a distinct luminescent part of interstellar medium, which can consist of ionized, neutral or molecular hydrogen and also cosmic dust. Nebulae are often star-forming regio ...
e. The effects and uses of the telecompressor are largely opposite to those of the teleconverter or
Barlow lens The Barlow lens, named after Peter Barlow, is a diverging lens which, used in series with other optics in an optical system, increases the effective focal length of an optical system as perceived by all components that are after it in the system. ...
. A combined system of a lens and a focal reducer has smaller
back focus The human back, also called the dorsum, is the large posterior area of the human body, rising from the top of the buttocks to the back of the neck. It is the surface of the body opposite from the chest and the abdomen. The vertebral column runs ...
than the lens alone; this places restrictions on lenses and cameras that focal reducer might be used with.
Lens adapter In photography and videography, a lens adapter is a device that enables the use of camera and lens combinations from otherwise incompatible systems. The most simple lens adapter designs, passive lens adapters provide a secure physical connectio ...
s that include telecompressors are useful with digital
mirrorless A mirrorless camera is a photo camera featuring a single, removable lens and a digital display. The camera does not have a reflex mirror or optical viewfinder like a digital single-lens reflex (DSLR) camera, but may have an electronic vi ...
cameras. By combining a telecompressor within a lens adapter, mirrorless cameras can use the lenses of both
digital single-lens reflex camera A digital single-lens reflex camera (digital SLR or DSLR) is a digital camera that combines the optics and the mechanisms of a single-lens reflex camera with a digital imaging sensor. The reflex design scheme is the primary difference between ...
s (DSLRs) and film-based SLR (
Single-lens reflex camera A single-lens reflex camera (SLR) is a camera that typically uses a mirror and prism system (hence "reflex" from the mirror's reflection) that permits the photographer to view through the lens and see exactly what will be captured. With twin le ...
s). Canon EF The EF lens mount is the standard lens mount on the Canon EOS family of SLR film and digital cameras. EF stands for "Electro-Focus": automatic focusing on EF lenses is handled by a dedicated electric motor built into the lens. Mechanically, it i ...
lenses with Micro Four Thirds camera bodies" mode="packed"> Telekompressor.kameragehaeuseseitig.jpg, Mount for Micro Four Thirds camera bodies with eleven gold contacts Telekompressor.seitig.jpg, Side view with aperture control rocker (above optical axis) and USB socket with rubber cover (below optical axis) Telekompressor.objektivseitig.jpg, Mount for Canon EF lenses (full-format) with eight gold contacts


Calculating focal reduction

For a
refractor A refracting telescope (also called a refractor) is a type of optical telescope that uses a lens (optics), lens as its objective (optics), objective to form an image (also referred to a dioptrics, dioptric telescope). The refracting telescope d ...
telescope A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to observe ...
or simple camera lens, the new effective focal length ''fn'' is given by: : = \left(1 - \frac\right), where ''fo'' = original focal length of telescope, ''d'' = distance from telecompressor to image plane, and ''fr'' = 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 photography and optics, vignetting is a reduction of an image's brightness or saturation toward the periphery compared to the image center. The word ''vignette'', from the same root as ''vine'', originally referred to a decorative border ...
in the image, depending on the sizes of the secondary mirror and the telescope tube. For a catadioptric system that has a combination of mirror and lens, the determination of reduction is more complicated, due to the fact that the telescope has a variable focal length, where the imaging plane can move along the axis of the imaging system. As the addition of the telecompressor will increase the necessary back focus, the original focal length will increase by a certain amount, and then this new focal length would be used in the above formula.


Keplerian (relay) telecompressors

Telecompressors were used in early digital SLR systems like the
Minolta RD-175 The Minolta RD-175 was an early digital SLR, introduced in 1995. Minolta combined an existing SLR with a three way splitter and three separate CCD image sensors, giving 1.75 megapixel (MP) resolution. The base of the DSLR was the Minolta Maxxum ...
and the
Nikon E series The Nikon E series, co-developed with Fujifilm, are autofocus 1.3 megapixel professional grade quasi- full frame (35mm) Nikon F-mount digital single lens reflex cameras (DSLR) manufactured by Nikon since 1995. The E series included the ''Nikon E2 ...
. The technology of the time used relatively small sensor sizes, so lenses designed for 35 mm 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, which means that a lens meant for a larger format can be used on a smaller sensor with a higher crop factor.


See also

*
Barlow lens The Barlow lens, named after Peter Barlow, is a diverging lens which, used in series with other optics in an optical system, increases the effective focal length of an optical system as perceived by all components that are after it in the system. ...
* Teleconverter *
Convertible lens A camera lens (also known as photographic lens or photographic objective) is an optical lens or assembly of lenses used in conjunction with a camera body and mechanism to make images of objects either on photographic film or on other media capab ...


References

Astronomical instruments Photography equipment {{film-tech-stub