Canon ST-E2 Speedlite Transmitter for Canon 580EX II, 430EX, 430EX II Speedlites
The Canon Speedlite Transmitter ST-E2 is compatible with Type A EOS  cameras (E-TTL, auto flash) and Type B EOS cameras (manual flash). The ST-E2 is  designed to control slaved Canon 550 EX flashes, enabling simultaneous flash use  in professional settings. The wireless controller has a flash ratio control,  allowing for several different A:B ratios, 1:8 to 8:1 in half-step increments,  as well as slave confirmation via a test transmission button, which sets off  slave A, followed by slave B, at 1/64 output.