ST-E10 flash controller

The R3 is compatible with the new ST-E10 flash controller. Unlike the ST-E3-RT, the ST-E10 can command manual flashes down to 1/8192 flash energy. This is supported by the very expensive EL-1, for example. For older flashes, like the 600EX-RT and 600EX II-RT, their manuals state minimum 1/128.
However, if the old flash is commanded to emit only 1/8192, the resulting flash is clearly weaker than what you get at 1/128. Does anybody know how far down the 600 EX-RT really is working? It's too bright to test with any significant result here now, or I could find out myself.

Now when it got dark I can answer my own question.
When commanded from an ST-E10 flash controller on an R3 camera, the older 600EX-RT flashes will indeed accept manual flash power settings down to 1/4096 of full power, not just down to 1/128.

Thanks for the info. At some point I need to build out my flash system a bit more and it's good to know how new I need to go to get the latest advantages. Makes me feel a bit behind the times when the 600EXII-RT is considered an older flash...

Don't worry, I don't have anything newer either!

Note that this is not according to what Canon's manuals state. They claim you can't set these flashes below 1/128 manually, no matter what.
The ST-E3-RT version 2 can also command to lower levels (your old ST-E3-RT can be upgraded, for a fee), but they still claim it's only for EL-1 and similar.