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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 Lens speed refers to the maximum aperture diameter, or minimum f-number, of a photographic lens. A lens with a larger than average maximum aperture (that is, a smaller minimum f-number) is called a "fast lens" because it can achieve the same exposu ...
, and in some instances improve
optical transfer function The optical transfer function (OTF) of an optical system such as a camera, microscope, human eye, or projector specifies how different spatial frequencies are captured or transmitted. It is used by optical engineers to describe how the optics pro ...
(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 Videography is the process of capturing moving images on electronic media (e.g., videotape, direct to disk recording, or solid state storage) and even streaming media. The term includes methods of video production and post-production. It used ...
, and
astrophotography Astrophotography, also known as astronomical imaging, is the photography or imaging of astronomical objects, celestial events, or areas of the night sky. The first photograph of an astronomical object (the Moon) was taken in 1840, but it was no ...
. 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 A teleconverter (sometimes called tele extender) is a secondary lens mounted between a camera and a photographic lens which enlarges the central part of an image obtained by the lens. For example, a 2× teleconverter for a 35 mm camera enl ...
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 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 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 as its objective to form an image (also referred to a dioptric telescope). The refracting telescope design was originally used in spyglasses and a ...
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 A reflecting telescope (also called a reflector) is a telescope that uses a single or a combination of curved mirrors that reflect light and form an image. The reflecting telescope was invented in the 17th century by Isaac Newton as an alternati ...
, 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 A secondary mirror (or secondary) is the second deflecting or focusing mirror element in a reflecting telescope. Light gathered by the primary mirror is directed towards a focal point typically past the location of the secondary. Secondary mirro ...
and the telescope tube. For a
catadioptric A catadioptric optical system is one where refraction and reflection are combined in an optical system, usually via lenses (dioptrics) and curved mirrors ( catoptrics). Catadioptric combinations are used in focusing systems such as searchlights, ...
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 An axis (plural ''axes'') is an imaginary line around which an object rotates or is symmetrical. Axis may also refer to: Mathematics * Axis of rotation: see rotation around a fixed axis * Axis (mathematics), a designator for a Cartesian-coordinat ...
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 E ...
. 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 The image circle is the cross section of the cone of light transmitted by a lens or series of lenses onto the image plane. When this light strikes a perpendicular target such as photographic film or a digital camera sensor, it forms a circle of ...
, which means that a lens meant for a larger format can be used on a smaller sensor with a higher
crop factor In digital photography, the crop factor, format factor, or focal length multiplier of an image sensor format is the ratio of the dimensions of a camera's imaging area compared to a reference format; most often, this term is applied to digital ca ...
.


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 A teleconverter (sometimes called tele extender) is a secondary lens mounted between a camera and a photographic lens which enlarges the central part of an image obtained by the lens. For example, a 2× teleconverter for a 35 mm camera enl ...
*
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 capa ...


References

Astronomical instruments Photography equipment {{film-tech-stub