Teleconverter

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).

A teleconverter attached between a camera and its lens
An object in the image plane appears smaller by the reciprocal of the teleconverter's rating when looked through it.
An Olympus DSLR with a 2× Olympus EC-20 teleconverter. [ 1 ]
1 – Camera lens
2 – Teleconverter
3 – Camera body
Teleside converter cross section