To ask questions of the Photo Video Guy, send your question in an email to firstname.lastname@example.org I did a seminar on getting your flash off camera at my local Henry's store yesterday and Dennis asked if I had a solution for the SL1. He was curious if the Speedlite 90EX could be used as a master flash.
The SL1 has a popup flash but it doesn't have the capability to control other Canon flashes via optical transmission as is found in the T3i and higher cameras. There are other cameras, like the older 30D that have a popup without this functionality either.
Dennis said that he had heard that the Speedlite 90EX could be a master and be used to control other Canon Speedlites acting as slaves.
He's mostly correct. The Speedlite 90EX can be a master controller, but not a Master Flash. If you go into the External Flash Control menus, and turn wireless on with the 90EX mounted and powered up, it becomes a controller only. Sort of like the ST-E2 device. It's no longer capable of being used as a flash. So long as the camera has the ability to control an external EX series flash from the camera menu system, that flash can be a master controller. In the case of the 90EX, this works but turns off the flash function of the 90EX.
Which is just fine after all since you don't want it contributing to the exposure anyway. I could not put hands on a 90EX to try on an SL1 but according to the documentation on both sides, this will work. Which is cool since a 90EX is only about $130 whereas the ST-E2 is discontinued and even used they are selling for $180.
I'm going to try to borrow a 90EX (it comes in the kit with the EOS-M) and test this out live and will update this post if it fails.