The kenko 14x teleplus hd dgx teleconverter contains a set of optics which multiply the focal length of your prime lens by 14x turning a 300mm lens into the equivalent of 420mm teleplus hd dgx converters use unique circuitry which maintains signal integrity between the camera body and lens and it also allows accurate recording of digital exif data full af operation with the hd dgx 14x is possible using a lens with an aperture of f4 or brighter except canon ef 50mm f18 please be aware t