The KIWIFOTOS 2x Telephoto Converter is for use with T-Mount lenses, mounting between the camera and a photographic lens. The converter delivers a double increase to the focal length of your existing lens, and cuts back the amount of light by two f/stops. (For example, using a 2x tele-converter with a 500mm lens will effectively increase the focal length to 1000mm. A lens set to f/2.8 will now be f/5.6. ) Auto focusing is not supported with this tele-converter.