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I'll give that a shot Ian. It's just the the 580EXII is completely unreliable as a Master, as it often switches itself to TTL and fires full power.
Unfortunately it does seem that it is somehow caused by an incompatibility with the RPs and 1DX, as it works every time on the 5D Classic.
Question about the 600EX: Can I trigger a second 600EX with radio, and an older 430EX optically, at the same time in a setup? Also, can I use the 600EX's radio system on a 5D Classic. I would assume so, as it should just be the flashes communicating with each other here, correct?