Developed in conjunction with the Speedlite 550EX and the EOS-3 camera, this master control device for a wireless Speedlite flash system meets professional and advanced amateur standards. Its dedicated transmitter controls up to 3 Canon 550EX Speedlites as slave units 33ft. away outdoors and 49.5ft. away indoors. It is mounted as a clip-on with shoe lock lever and locking pin.