The kenko teleplus hd dgx 20 teleconverter contains a set of optics which multiply the focal length of your prime lens by 2x turning a 300mm lens into the equivalent of 600mm teleplus hd dgx converters use unique circuitry which maintains signal integrity between the camera body and lens it also allows accurate recording of digital exif data multi coated optical glass is used in the optics which is supplied by hoya corporation the worlds largest manufacturer of optical glass full af ope