Teleconverter - Function

Function

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.

  • Camera viewfinder with 300mm telephoto lens.

  • Camera viewfinder with 300mm telephoto lens and 2x teleconverter.

  • A Leica R series doubler, with the female part of in bayonet mount...

  • ...and the male part.

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