A teleconverter (sometimes called tele extender) is a

secondary lens In photography, a secondary lens or accessory lens is a lens designed to be used in conjunction with another lens, called the ''primary lens''. A secondary lens may be designed to be used either in front of the primary lens, between it and the ...

mounted between a

camera A camera is an optical instrument that can capture an image. Most cameras can capture 2D images, with some more advanced models being able to capture 3D images. At a basic level, most cameras consist of sealed boxes (the camera body), with ...

and a

photographic 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 ...

which enlarges the central part of an image obtained by the lens. For example, a 2× teleconverter for a 35 mm camera enlarges the central 12×18 mm part of an image to the size of 24×36 mm in the standard 35 mm

film format A film format is a technical definition of a set of standard characteristics regarding image capture on photographic film for still images or film stock for filmmaking. It can also apply to projected film, either slides or movies. The primary c ...

. Teleconverters are typically made in 1.4×, 1.7×, 2× and 3× variants, with 1.4× and 2× being the most common. A 2× teleconverter doubles the apparent

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 foc ...

of a given lens. Teleconverters also decrease the intensity of the light that reaches the film plane (or sensor) by the square of its magnification. A 2× teleconverter reduces the light to 1/4, doubles the focal ratio and halves the resolution of the master lens it is connected to. This, however, does not necessarily halve the resolution of the digital image. A closely related device reduces the focal length. It is generally marketed under the name

speed booster 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. It is also widely known under the name “Speed Booster”, which is ...

, though the term

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. It is also widely known under the name “Speed Booster”, which is ...

has also been used. It is used in particular to adapt

DSLR 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 a ...

lenses to mirrorless cameras. Here the magnification is less than 1, typically 0.71× or 0.64×. In this case, the effective aperture increases accordingly.

Function

A teleconverter works similarly to a ''telephoto group'' of a proper

telephoto lens A telephoto lens, in photography and cinematography, is a specific type of a long-focus lens in which the physical length of the lens is shorter than the focal length. This is achieved by incorporating a special lens group known as a ''telephoto ...

. It consists of a group of lenses which together act as a single diverging lens. The location of a teleconverter is such that the image produced by the lens is located behind the teleconverter in a distance smaller than its focal length. This image is a ''virtual object'' of the teleconverter, which is then focused further away and thus enlarged. For example, when a single negative lens is placed so that the image formed by the lens is located in the midpoint between the lens and its

focal point Focal point may refer to: * Focus (optics) * Focus (geometry) * Conjugate points, also called focal points * Focal point (game theory) * Unicom Focal Point UNICOM Focal Point is a portfolio management and decision analysis tool used by the p ...

, then the lens produces the image in its focal point and enlarging it two-fold, thereby acting as a 2× teleconverter. When used with somewhat slow lenses they may reduce the effective aperture enough that the camera's autofocus system will no longer work; depending on the camera system, this may range from f/5.6 to f/8. Dedicated teleconverters only work with a limited number of lenses, usually telephoto lenses made by the same manufacturer, or by a third-party manufacturer to a matching standard. Using a teleconverter with an existing lens is usually less expensive than acquiring a separate, longer telephoto lens, but as the teleconverter is magnifying the existing image circle, it also magnifies any aberrations. The use of a teleconverter also results in a darker image. File:Viewfinder-SLR-300mm.swn.jpg, Camera viewfinder with 300 mm telephoto lens. File:Viewfinder-SLR-300mm-2xTeleconv.swn.jpg, Camera viewfinder with 300 mm telephoto lens and 2x teleconverter. File:Leica-doubleur-p1020785.jpg, A Leica R series doubler, with the female part of in

bayonet mount A bayonet mount (mainly as a method of mechanical attachment, such as fitting a lens to a camera using a matching lens mount) or bayonet connector (for electrical use) is a fastening mechanism consisting of a cylindrical male side with one ...

... File:Leica-doubleur-p1020786.jpg, ...and the male part. File:Nikon TC-14EII teleconverter.jpg, Nikon AF-S teleconverter TC-14E II File:Nikon Z-mount teleconverter 2.0x.jpg, Nikon teleconverter 2,0x for

Z-mount Nikon Z-mount (stylised as \mathbb) is an interchangeable lens mount developed by Nikon for its mirrorless digital cameras. In late 2018, Nikon released two cameras that use this mount, the full-frame Nikon Z 7 and Nikon Z 6. In late 2019 N ...

File:Vivitar zoom teleconverter.jpg, Vivitar 2X zoom teleconverter, basically a zoom

extension tube An extension tube, sometimes also called a closeup tube or an extension ring, is used with interchangeable lenses to increase magnification. This is most often used in macro photography. Construction The tube contains no optical elements; it ...

Teleside converter

A different type of teleconverter called a teleside converter''Scientific photography and applied imaging'' By Sidney F. Ray, page 492
/ref> can be mounted on the front of the camera's lens rather than between the primary lens and the camera body. These are popular with users of

video camera A video camera is an optical instrument that captures videos (as opposed to a movie camera, which records images on film). Video cameras were initially developed for the television industry but have since become widely used for a variety of oth ...

s and

bridge camera Bridge cameras are cameras that fill the niche between relatively simple point-and-shoot cameras and interchangeable-lens cameras such as mirrorless cameras and single-lens reflex cameras (SLRs). They are often comparable in size and weight t ...

s with fixed lenses, as they represent the only way to add more reach to such a camera. They are usually afocal lenses that do not reduce the brightness of the image, but are more likely to add aberrations to the image, independent of the quality of the main lens.

Versus extension tubes

Teleconverters may be confused with

s, a non-optical component designed to increase magnification (at the expense of reduced focal distance).

References

{{reflist Photographic lenses

Function

Teleside converter

Versus extension tubes

See also

References