Flashes don't measure light (or at lest not in E-TTL. They did it in A-TTL). They just pre-fire in their group. Then the camera measures the light and tells the group how strong it should fire.550EX will work on even newest camera, just wont measure light according to latest standard.
Groups D and E are radio-only and likely aren't supported by 600D even with an RT master.
Upvote
0