With the Speedlite 550EX and the Speedlite Transmitter ST-E2, it is now possible to use multiple Speedlites under full E-TTL control and without the need for any interconnecting wires. 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 33 feet away outdoors and 49.5 feet away indoors. It is mounted as a clip-on with a shoe lock lever and locking pin.