The Canon ST-E2 Speedlite Transmitter is a dedicated E-TTL (II) wireless autoflash transmitter that can control an unlimited number of flashes with Canon Speedlites 420EX, 430EX, 430EXII, 550EX, 580EX and 580EX in two separate groups; A and B. The ST-E2 sets and controls all functions of its slave units. The flash output ratio between groups A and B can also be adjusted from 8:1 to 1:1 or from 1:1 to 1:8. Under low-light or low-contrast conditions it emits an AF-assist beam linked to the camera's focusing points.