Upload & Sell: Off
I have 2 Flash heads Canons 550EX and a 600EX-RT. I'm thinking about buying another 600EX-RT and Canon ST-E3-RT Speedlite Transmitter radio controller to trigger the 600s. Can I trigger the 550EX as a slave via optical in this configuration?
The 600EX-RT can operate via radio or optical wireless (but not both at the same time), but the ST-E3-RT is radio only and the 550EX is optical only.
The only way to use the 550EX with the ST-E3-RT would be to trigger it in Manual mode using an optical slave. It would then trigger off the flash from the 600EX-RTs.
The latter would have to be in Manual mode as well, because in ETTL mode there would be metering preflashes that would prematurely trip the 550EX. (There are "smart" optical triggers that can be set to ignore a certain number of preflashes, but I have little experience with them.)
Manual mode is a good way to work much of the time, though, and you could still control the 600EX-RTs remotely from the ST-E3-RT (including using the camera's flash control menus if it's a newer camera.)