Teleconverters also decrease the intensity of the light that reaches the film plane (or sensor) by the square of its magnification.
A 2× teleconverter reduces the light to 1/4, doubles the focal ratio and halves the resolution of the master lens it is connected to.
It consists of a group of lenses which together act as a single diverging lens.
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.
Teleconverters may be confused with extension tubes, a non-optical component designed to increase magnification (at the expense of reduced focal distance).