Canon Speedlite Transmitter ST-E2

Type: 550EX-dedicated, clip-on Speedlite transmitter with direct contacts
Compatible Cameras: Type A EOS cameras (E- TTL, autoflash); Type B EOS cameras (Manual flash)
Channels: 4
Flash Ratio Control: For A:B ratio: 1:8 to 8:1, in half- step increments or 13 steps
High- Speed Sync (FP flash): Enabled with high-speed sync mode
Slave Operation Confirmation: With test transmission button. (Slave A fires followed by slave B at 1/64 output.)
Flash Exposure Confirmation:

Before flash fires during FE lock Adequate flash exposure indicated by the flash exposure level icon lit in the viewfinder. Insufficient flash exposure indicated by the flash exposure level icon blinking in the viewfinder.
After flash fires ST-E2's flash confirmation lamp lights in green for 3 sec.

Wireless slave SE mode cancellation: Cancels when the test transmission button or FE lock botton is pressed to turn on the slave unit.
Wireless Transmission: Infrared pulse
Transmission Range: Indoors: Approx. 12-15 meters/39.4 - 49.2 ft; Outdoors: Approx. 8-10/ 26.2-32.8 ft
Transmission Coverage: ±40° horizontal and ±30° vertical

Canon Speedlite Transmitter ST-E2, 2.56" (65 mm), 2.01" (51 mm), 3.15" (80 mm), 3.53 oz (100 g)