The Canon 2478A002 ST-E2 Speedlite Transmitter is a dedicated clip-on transmitter for E-TTL / E-TTL II wireless flash system. The auto flash transmitter can control an unlimited number of 420EX, 430EX, 550EX and 580EX flashes, in two separate groups; A and B. It 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 EOS-3's 45 focusing points.
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.