The ST-E3-RT is radio-only. So while the 600EX II RT flashes can do either radio or optical trigger... since the ST-E3-RT only does radio, the flashes only work in radio mode with that transmitter.
It's nice that the flashes have optical because this lets them work with the older optical-only flashes.
Both flashes must be the mode with the radio icon and slaves. As I recall, the master supports up to 15 slaves.
There are a few more things you need to know....
First, radio spectrum can be congested and normally the flashes try to auto-select the base radio frequency to use for communication. But it turns out you can also manually pick the channel. The important thing is that all devices must be using the same channel. So if you manually set a channel on the transmitter... make sure the slaves are using the same channel or they'll never receive the messages. In most situations you can just use "auto" as the channel and it will search for a good one.
Next, even with different channels available, Canon realizes that it still can be posssible for you to be at an event where someone else owns the same system... and happens to be using the same channel. So Canon ALSO lets you specify your own ID number. This is a 4 digit number (when you get a new flash or transmitter the ID number is set to "0000" as a default). Every device in your flash system must use the same ID number of they wont be able to talk to each other (even if they are on the same channel). Think of this as your own private flash club... these flashes will only talk to other flashes that are members of the same club. So if you set yoru ID number as 1234 on one component (such as the master) then EVERY other device that you plan to use (all the slave flashes) must also set the ID to 1234 (the numbers must be the same.) This prevents other photographers from triggering your flashes and vice versa.
...and then there are "groups"
By default the group is set to ALL (meaning you aren't using groups - they all fire as one). But since you want to use one flash as a "key" light and another as a "fill" light you don't want them to fire at the same power - you want to control these independently. So you can set one flash to "group A" (even though there's only one flash in the group) and another to "group B".
Having done this, you can tell the master that want use Group "A:B" or "A+B". The slight nuance is that the colon means they will be used as a "ratio" so you can set Group A to fire at double the power of Group B for instance and the flash system will maintain the ratio. If you use the "A+B" then it fires both groups and you set the power level of each "group" indendent of the other (it doesn't have to pay attention to ratios).
The system is quite powerful once you learn it's capabilities.
You may want to pick up a copy of "Speedliter's Handbook" by Syl Arena. It's probably the best book on the Canon flash system (the book is Canon specific).
Tim Campbell
5D III, 5D IV, 60Da
