. These devices are pretty expensive, and some of the newer Canon aps-c camera's have a flash-master in the built-in flash - so check this before considering an ST-E2! Also note the newer flashes, like the 600RT have a radio-based trigger, as opposed to optical trigger.
- The ET-S2 supports 4 different channels to avoid interference from other photographers equipment
- Within this frequency the ST-E2 supports 2 groups (A and B), though not group C (which is supported by the speedlites).
- You can have multiple speedlite's within a group (each group will be remotely triggered simultaneously)
- You can configure the balance between the two groups (strength-factor between 1:1 or up to 1:8)
- works on infrared
With the Canon 550D you have a depth-of-field preview (button on the left-hand side, front of the camera). This also triggers a 1 second preview of the light (the speedlite's will flash continuously for 1 second). Note that this makes the flashes run hot, so don't do it more then a few times (or the flash will trigger the 'heat-protection' mechanism and shut itself down for some time).
My (very patient) test-subject with a speedlite flash on each side
The camera (EOS550D or Rebel T2i) has the ST-E2 installed in the shoe. The speedliteon the left (canon 430exII) is set to channel 1, group A
The speedlite on the right (canon 430ex) is set to channel 1, group B
ET-S2 with a ratio of 8:1
Notice the strong left flash compared to the weak right flash - and the shadow on the right part of the wall.
ET-S2 with a ratio of 1:8
Notice the strong right flash compared to the weak left flash - and the shadow on the left part of the wall
ET-S2 with a ratio of 1:1
Both flashes firing on the same strength - the shadow on the wall is on both the left and the right side
This kind of setup is used for portrait photography. If you use the on-camera flash you will get hard shadows behind your subject. With the remote triggering using the ST-E2, you can let the light come from one side and fill in with a bit of light from the other side. This avoids the hard shadows which can fall on wall behind your subject when using an on-camera flash.