A teleconverter (or doubler) 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 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×, 2× and 3× models.
The use of a 2× teleconverter gives the effect of using 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).
 Camera viewfinder with 300mm tele lens... |
 Camera viewfinder with 300mm tele lens and 2x teleconverter |
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.
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.
 A Leica R series doubler, with the female part of in bayonet mount... |
 ...and the male part. |
A different type of teleconverter 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 bridge cameras with fixed lenses, as they represent the only way to add more reach to such a camera. They 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 objective.
See also
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