Teleconverter

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 enlarges the central 12×18 mm part of an image to the size of 24×36 mm. Teleconverters are typically made in 1.4×, 2× and 3× models.

The use of a 2× teleconverter (or doubler) gives the effect of using lens with twice the focal length. It also decreases the intensity of the light reaching the film by the factor of 4 (an equivalent of doubling the focal ratio) as well as the resolution (by the factor of 2).

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.

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 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 [ [http://www.raynox.co.jp/english/egindex.htm] , website of front-mounted converter manufacturer] . Such "afocal" [ [http://www.cs.mtu.edu/~shene/DigiCam/User-Guide/FZ-30/Converters/FZ-30-overview.html] , Ching-Kuang Shene, Michigan Technological University] conversion lenses are frequently discussed on Bridge digital camera boards. [ [http://www.flickr.com/groups/panasoniclumixdmcfz18] , e.g., Flickr Panasonic Lumix DMC FZ18 discussion board] .

ee also

*Barlow Lens
*Canon Extender EF

References