Tele-converters are additional devices used between lens and camera and to increase the focal length of the mounted lens. The number stated in the name typically 2x or 1,4x represents the factor by which the focal length of the mounted lens is increased, e.g., a 2-fold Tele-converter doubles the focal length of the lens. Tele-converters are often an inexpensive alternative to an additional lens. The disadvantage however is that that the luminosity of the mounted lens decreases in the same relation to the increase in focal length. The use of tele-converters is not recommended with long focal length lenses, wide-angle lens and so-called super zooms, as the image notably decreases.
Tele-zooms are lenses with a variable focal length starting from an angle of view of approx. 20 °. This lens class is suited for capturing distant motives up close, e.g., sports, animal or theatrical photography. But they are also classically used for portraits. With this lens class the frame can be changed with a shift in the focal length (zoom).