The Canon Speedlite Transmitter ST-E2 is a small and light Canon wireless slave flash controller and focus assist device.
Need to trigger slave (remote) Canon Speedlite Flashes? You can use a Canon Speedlite Flash with the master capability including the Canon Speedlite 580EX Flash and the Canon Speedlite 550EX Flash. Or you can use the Canon Speedlite Transmitter ST-E2.
There are some other advantages to using a flash as the master. While both the Canon Speedlite Transmitter ST-E2 and the flashes will simultaneously master any number of slave (remote) flashes (including the 580EX, 550EX, 430EX and 420EX), the Canon Speedlite Transmitter ST-E2 controls only 2 channels (A and B) while the 550EX and 580EX flashes will control 3 channels (A, B and C). Both the Canon Speedlite Transmitter ST-E2 and the flashes will allow flash output ratios to be adjusted between the channels they control. The ST-E2 allows ratios from 8:1 to 1:1 to 1:8 to be selected in 1/2 stop increments. I expect that 2 channels are enough for most people. Flash output at the subject can also be controlled by moving them farther away and/or adding diffusers.
Both the Canon Speedlite Transmitter ST-E2 and the flashes will provide focus assist. You might want to find out how many focus points each supports. While the Canon Speedlite Transmitter ST-E2 and 550EX Flash support 45 focus point bodies, the 580EX supports all Canon EOS body focus points. The ST-E2 focus assist effective distance ranges from 30' in the center to 16.5' in the periphery. EF lenses 28mm and up are supported, AI Servo mode is not supported.
The 550EX and 580EX master flashes can enable FEB (Flash Exposure Bracketing) and allow manual flash output to be controlled by the master - Two capabilities not found in the Canon Speedlite Transmitter ST-E2. The Canon Speedlite Transmitter ST-E2 and the master flashes all support highspeed flash.
There are advantages to using the Canon Speedlite Transmitter ST-E2. The first is the size and weight. The Canon Speedlite Transmitter ST-E2 weighs only 3.5 oz without the non-included-in-the-box 2CR5 6v battery. The ST-E2 is very small compared to master flashes and mounts on an EOS body's hot shoe (or on a Canon Off Cammera Shoe Cord OC-E3 for extra creativity). It is really nice to have significantly less weight in your hand for a long shoot.
The Canon Speedlite Transmitter ST-E2's flash master effective range is 39-49' indoors and 26-32' outdoors. Used indoors, direct line of sight is often not required as the light bounces from walls and other objects to reach the flash units.
I mentioned that the Canon Speedlite Transmitter ST-E2 does not include the 2CR5 6v battery in the box - Take note of this fact when you place your order. The battery lasts about 1,500 transmissions at room temperature. Unless you count your shots, you should probably keep a spare on hand because the battery will surely fail when you need it most.