Sample Wireless Flash System You can easily set up an E-TTL autoflash system with the 550EX, ST-E 2 or MR-14EX as the sender unit and the 420EX as the receiver unit. Just set the 420EX's wireless selector to <receiver> and position the receiver unit(s) as desired within the wireless transmission range
In the same way as normal exposure compensation, you can set exposure compensation for flash. The flash exposure compensation amount can be set up to +/-3 stops in 1/3-stop increments. (If the camera's exposure compensation is set to 1/ 2 -stop increments, flash exposure compensation will be in 1/ 2 -stop increments.)
Options include the ability to: Fire one or more Speedlites off-camera and all at the same output, with or without the built-in flash firing Fire two or more Speedlites at different outputs, using A:B ratio control, with or without the built-in flash firing independently