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I think the 600EX-RT has radio trigger and it won't work with current 580EX, 580EX II, 430EX and 430 EX II, right?
Gosh.... I don't want to replace my whole setup... It costs too much.
The 600 just adds radio triggering to the puzzle, it is fully backward compatible with previous speedlites that use Infrared triggering; like the 580, 430, etc. series...no worries, you can mix and match, you just get greater range between speedlites with the 600 and you don't have to worry about line-of-sight issues like you do with an infrared beam/trigger!