The Canon ST-E2 is Canon's dedicated wireless flash transmitter. Canon provides two different means to wirelessly control your remote Speedlites today. The first is to use a 580EX II Speedlite as a “master” and the second is to use Canon’s Speedlite Transmitter ST-E2.
There are some distinct advantages and disadvantages to using the Canon Speedlite Transmitter ST-E2. The biggest advantage is its size and weight. It is very small compared to the 580EX II and mounts on your DSLR’s hot shoe. The size and weight savings can be a real advantage when shooting on location.
Both the ST-E2 and the 580EX II master will allow flash output ratios to be adjusted between the channels they control with the ST-E2 allowing ratios from 8:1 to 1:1 to 1:8 to be selected in 1/2 stop increments. And while both the Canon Speedlite Transmitter ST-E2 and the 580EX II will support any number of remote flashes, the ST-E2 controls only 2 channels while the 580EX II will control all three. Another big advantage to using a 580EX II as a master is the effective range. The ST-E2′s effective range is only about 40 feet indoors and 30 feet when used outdoors. The range of the 580EX II master is more than twice that.
Contrary to some reviews posted on the net, the Canon Speedlite Transmitter ST-E2 does support manual flash output. However, you need to manually adjust power levels on your off-camera flashes, typically taking multiple test shots to test exposure and light levels, and adjusting the flash output iteratively until you have the right balance.
On the other hand, you can let the camera and the ST-E2 do all the hard work, and control the flashes automatically using wireless E-TTL (thru-the-lens metering). Flash exposure compensation on the camera can be used to adjust the flash exposure, or you can use the ratio functionality of the ST-E2 to adjust flash output relative to the flash output automatically chosen by the camera.
The ST-E2 is a great accessory. At only $220 (USD) it’s less expensive than using one of my 580EX II’s as a master and its way less expensive than buying a set of PocketWizards. I’ve never found “line-of-sight” to be a problem when used indoors as the infra red bounces off the walls (ST-E2 uses infra red light to communicate with the flash). There is however a slight challenge when shooting outdoors as the “line-of-sight” between the ST-E2 and the flash has to be maintained.
Note that the Canon 7D body has built-in wireless flash capability, and uses its built-in flash to provide similar functionality as the ST-E2. I however hate the fact that the pop up flash on the camera has to be on for this function to work. The more reason I went for the ST-E2. I hope the new 7D MK II will sort out this issue.