The above named speedlights are the latest and greatest in the Canon speedlight range and a little advice to the other Canon users that might save you some cash.
Only purchase if you have a 2012 or later camera.
I bought one of these to add a second light for portrait work and tried using it grouped with my older speedlight, a Canon 430EXII fired by Pixl King radio transmitters. I also have a very old optical slave speedlight which i add into my mix
Canon cameras allow you to control groups of speedlights from the LCD screen when attached via compatible radio transmitters, and I’d been using mine with 2 speedlights for quite a while but with the new 600′s only one speedlight was showing on screen. I could fire off two, but the settings were being mirrored on each flash instead of how i had used my older ones previously, setting the power to each independently.
I had my new flash checked and it was returned with a note saying it worked perfectly. I blamed my old flash, so stupidly went out and bought another 600, but still the same issue.
I had my 5DMKii checked, it too was in good nic and wasn’t the cause.
A check on good old Youtube brought the answer. These new flashes are only fully compatible with newer cameras in radio mode and don’t function properly with 2nd party radio transmitters, only the Canon built STE3 and only on newer bodies.
My dilema was get shot of the new flashes at a loss or upgrade my camera, which i did but it was an expensive lesson.
The flashes have their own inbuilt radio transmission/recievers and with one attached to the cameras hot shoe you can fully operate the 2nd but to get the full benefit you want to add the ST-E3 radio trigger unit which sits in the hot shoe and lets you set each flash output
You can pair ettl with manual etc but you can’t control the zoom remotely, that needs set on each unit manually.
Another negative the trigger doesn’t have any infrared AF assist, which I find quite bizarre. You can have that by making a switch and having one of the flash in the hot shoe but it kind of defeats the point in paying for the trigger, the only way to keep two flashes off camera in low light is to use a torch for helping with AF, set your focus in manual or get a hot shoe cable attachment and connect to the flashes that way.
An expensive lesson, but the flashes are great and grouping is now easy peasy.