Augmenting my 600 EX RT wireless flash system

OK the 7D has been replaced by the 5DMKIII and the radio based STE-3 RT; and I bought a single 600EXRT.
The plan is for off camera wireless, I can for quick set ups just use ETTL2 and one off camera flash.
For more demanding scenarios Ideally I will in the long term buy more 600s.
But for now I am hoping I can use the 600 as my main and either the 430mk II
Triggered by the 600s flash.
But this means it would be manual, and I need a Flash Zebra optical cell and a hot-shoe adapter total cost £25 ($39)

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If you add manually set flashes to a 600 EX-RT, operating in E-TTL II, then the camera will (hopefully) not see the manual flashes when it fires the pre-flash for E-TTL II metering.
Thus the additional light from the manual flashes will not be accounted for. It's your responsibility to make sure that it doesn't cause overexposure.
But this is no different from if you had used one or more additional 600 EX-RT flashes and set them manually through the group mode.
In fact, even if you have two or more groups of 600 EX-RT in E-TTL II mode, they'll be metered individually, and thus cause overexposure if they cover the same area.
The old mode, allowing three groups of flashes, can be used to automatically blend groups A and B onto the same subject.

Apart from that, triggering manual flashes from a 600 EX-RT requires that the manual flashes can disregard the pre-flash.

The YN-560s have an S1 and S2 mode. This permits them to ignore the preflash and then fire optically with the main flash. It will work with the 600 just as it does with other ETTL flashes. What I've never tested is if it will work when the 600 is being fired by the STE3. Theoretically it will work. Real world would require some testing. Ultimately more 600s are better than other alternatives. But the inexpensive manual speedlites will always be useful. The 430EXII is, in my opinion, obsolete. You're buying bits to turn it into a manual flash when you can get more light for less money other ways.

Since the control of the flash is via radio, the pre-flash has the same characteristics from a 600 EX-RT used as a slave flash (in E-TTL II mode), as it has when the flash is used independently in the camera's hot shoe.

It's not like Canon's optical control technology, where a multitude of flashes are sent prior to actual release for taking the picture.

apersson850 wrote in post #16116724
Since the control of the flash is via radio, the pre-flash has the same characteristics from a 600 EX-RT used as a slave flash (in E-TTL II mode), as it has when the flash is used independently in the camera's hot shoe.

It's not like Canon's optical control technology, where a multitude of flashes are sent prior to actual release for taking the picture.

Thats what I was hoping for.
My real concern was that the radio equivalent of the preflash is I hope, just the trial flash so it can be evaluated for ETTL2. Obviouslynthis single tial flash will I assume set off the YNs?
I guess its the timing of when the YNs go off , as to if they are correctly evaluated by the 600 and the camera.
Think this might merit buying one yn 560II to try it out.

The radios are used for the control communication. There is still a pre-flash for ETTL metering. That's the flash that sets off the YN-560 unless its set to S1 or S2. I don't remember which.

dmward wrote in post #16119884
The radios are used for the control communication. There is still a pre-flash for ETTL metering. That's the flash that sets off the YN-560 unless its set to S1 or S2. I don't remember which.

Thanks and great - that is what I expected.