Type: 550EX-dedicated, clip-on Speedlite transmitter with direct contactsCompatible Cameras: Type A EOS cameras (E- TTL, autoflash); Type B EOS cameras (Manual flash)Channels: 4Flash Ratio Control: For A:B ratio: 1:8 to 8:1, in half- step increments or 13 stepsHigh- Speed Sync (FP flash): Enabled with high-speed sync modeSlave 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 pulseTransmission Range: Indoors: Approx. 12-15 meters/39.4 - 49.2 ft; Ou