Teleconverter
Encyclopedia
A teleconverter is a secondary lens
which is mounted between the camera
and a photographic lens
. Its job is to enlarge the central part of an image obtained by the objective 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
. Teleconverters are typically made in 1.4×, 1.7×, 2× and 3× models, of which 1.4× and 2× are most common.
The use of a 2× teleconverter gives the effect of using a lens with twice the focal length
. It also decreases the intensity of the light reaching the film by a factor of 4 (an equivalent of doubling the focal ratio) as well as the resolution
(by a factor of 2).
A teleconverter works similarly to a telephoto group of a proper telephoto lens
. 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 objective 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 objective is located in the midpoint between the lens and its focal point
the lens produces the image in its focal point enlarging it two times thus 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
.
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
s and bridge cameras 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.
Secondary lens
In photography, a secondary 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 subject, or behind the primary lens, between it and the film.Secondary lenses...
which is mounted between the camera
Camera
A camera is a device that records and stores images. These images may be still photographs or moving images such as videos or movies. The term camera comes from the camera obscura , an early mechanism for projecting images...
and a photographic lens
Photographic lens
A camera lens 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 capable of storing an image chemically or electronically.While in principle a simple convex lens will suffice, in...
. Its job is to enlarge the central part of an image obtained by the objective 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
Film format
A film format is a technical definition of a set of standard characteristics regarding image capture on photographic film, for either stills or movies. It can also apply to projected film, either slides or movies. The primary characteristic of a film format is its size and shape.In the case of...
. Teleconverters are typically made in 1.4×, 1.7×, 2× and 3× models, of which 1.4× and 2× are most common.
The use of a 2× teleconverter gives the effect of using a lens with twice the focal length
Focal length
The focal length of an optical system is a measure of how strongly the system converges or diverges light. For an optical system in air, it is the distance over which initially collimated rays are brought to a focus...
. It also decreases the intensity of the light reaching the film by a factor of 4 (an equivalent of doubling the focal ratio) as well as the resolution
Angular resolution
Angular resolution, or spatial resolution, describes the ability of any image-forming device such as an optical or radio telescope, a microscope, a camera, or an eye, to distinguish small details of an object...
(by a factor of 2).
A teleconverter works similarly to a telephoto group of a proper telephoto lens
Telephoto lens
In photography and cinematography, a telephoto lens 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 group that extends the light path to create a long-focus...
. 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 objective 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 objective is located in the midpoint between the lens and its focal point
Focus (optics)
In geometrical optics, a focus, also called an image point, is the point where light rays originating from a point on the object converge. Although the focus is conceptually a point, physically the focus has a spatial extent, called the blur circle. This non-ideal focusing may be caused by...
the lens produces the image in its focal point enlarging it two times thus 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
Image circle
The image circle, or circle of illumination, of a lens is the circular area in the image plane formed by the cone of light transmitted by the lens . Within this circle is the smaller circle for which image definition is acceptable, the circle of good definition ; however, some authors make no...
, it also magnifies any aberrations
Aberration in optical systems
Aberrations are departures of the performance of an optical system from the predictions of paraxial optics. Aberration leads to blurring of the image produced by an image-forming optical system. It occurs when light from one point of an object after transmission through the system does not converge...
.
Teleside converter
A different type of teleconverter called a teleside converterTeleside converter
A teleside converter is a secondary lens which is mounted on the front of a photographic lens to increase the effective focal length of the lens they are attached to. They are used on cameras and video cameras with non–interchangeable lenses to increase the magnification of the image...
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
Video camera
A video camera is a camera used for electronic motion picture acquisition, initially developed by the television industry but now common in other applications as well. The earliest video cameras were those of John Logie Baird, based on the electromechanical Nipkow disk and used by the BBC in...
s and bridge cameras with fixed lenses, as they represent the only way to add more reach to such a camera. They are usually afocal
Afocal system
In optics an afocal system is an optical system that produces no net convergence or divergence of the beam, i.e. has an infinite effective focal length. This type of system can be created with a pair of optical elements where the distance between the elements is equal to the sum of each element's...
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 are sometimes confused with extension tubes, a non-optical component designed to increase magnification (at the expense of reduced focal distance).See also
- Barlow LensBarlow lensThe Barlow lens, named for its creator, the English engineer Peter Barlow, is a diverging lens which, used in series with other optics in an optical system, increases the effective focal ratio of an optical system as perceived by all components after it in the system...
- Canon Extender EFCanon Extender EFThe Extender EF lenses are a group of teleconverter lenses made by Canon. These lenses are used between any compatible EF type lens and any of the Canon EOS line of cameras. When used with a compatible lens, such as the Canon EF 500mm 4L IS USM, they will multiply the focal length of the lens by a...
- Nikon F-Mount TeleconverterNikon F-Mount TeleconverterThe Nikon F-Mount Teleconverter are a group of magnifying lenses mounted between the lens and the camera body. Currently there are 1.4x, 1.7x and 2x converters available.-Models:-Technical information:...
- TelecompressorTelecompressorA telecompressor is an optical element which amateur astronomers use to reduce the focal length of a telescope.Mainly used for astrophotography, the telecompressor gives a wider angle of view, a sharper picture, and reduces the exposure time needed. These are useful when taking pictures of nearby...