Canon Speedlite 600EX-RT with radio control
Finally has a major manufacturer of a DSLR system put the radio transceiver for communicating between DSLR body and flash where it belongs: Inside the flash itself. Congratulations to Canon for being first!
The Speedlite 600EX-RT flash, with an expected price tag of USD 629 is now in stock at B&H (there is even an USD 50 instant rebate until Sep. 12). The 2.4 Ghz two-way radio communication comes in addition to optical control. Like the Speedlite 580EX II, it can be used as both a master for other compatible Speedlites, and as a remote (slave) unit.
The new system do more than replace light signalling with radio for greater range and reliability. For the first time ever, remote flash units can call home and tell the master when they've recycled. This means that if you have remote Speedlites in awkward places, you no longer have to guess when they're ready again.
With the new radio-based system, the number of groups have been increased from 3 to 5. For this to work, you must use a Canon DSLR from 2012 (i.e. EOS 1D X, EOS 5D mark III or EOS 650D/T4i). With an older DSLR, you're limited to three groups.
The new flash is not more powerful that its predecessor, the Speedlite 580EX II. However its zoom head now goes from 20 mm to 200 mm coverage (35mm format). Its maximum guide number is higher, but the guide numbers for 24 mm to 105 mm are the same as the 580EX II. Compared to the 580EX II, the new flash has an improved AF assist beam that is compatible with Canon's new 61-point AF. The hot-shoe design has also been improved for increased dust and water resistance. A new gel holder, and a gel sensor, will make mounting of gels easier, and also ensure automatic adjustment of white-balance when a colour gel is used.
While Canon's flash control system has been extensively redesigned for the 600EX-RT, the new Speedlite is fully backwards compatible with the existing line of Speedlites. The new 600EX-RT can work in both radio mode and legacy optical mode (but not at the same time). There is no problem using the 600EX-RT with 580EX II, ST-E2, etc. as long as you stick with optical signalling in wireless mode.
For use as master on top of the camera, there is also a new Speedlite Transmitter ST-E3-RT. This new unit also uses Canon's new radio based flash control protocol. Unlike the flash, it does not support the legacy optical flash control protocol.