A teleconverter is a group of lenses 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. Typically there are 1.4×, 2× and 3× teleconverters found in stock.

The use of a 2× teleconverter gives the effect of using lens with twice the focal length. It also decreases the amount of 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.

The use of objective with teleconverter is less expensive than telephoto lens but also introduces some aberrations.