This is from an in depth article on Canon's site.
- Flash sync speed: When used for radio-based Wireless E-TTL with any Canon EOS camera introduced prior to the EOS 5D Mark III or EOS-1D X, the maximum available sync speed for flash will be one stop slower than normal. (EOS 5D Mark II or original EOS 5D – 1/100th second; EOS 7D, 60D, 50D, etc – 1/125th second; all previous EOS Rebel models – 1/100th second.)
This only applies when using radio-based wireless control of off-camera 600EX-RT Speedlites. If a 600EX-RT is simply used on-camera (alone), or if it's used for optical-based Wireless E-TTL in any way, the camera's normal, full flash sync speeds are available. - No high-speed sync: For radio-based wireless E-TTL, the only cameras that allow hi-speed flash sync will be the EOS 5D Mark III and EOS-1D X (as of March, 2012). It's not possible with any other previous EOS camera bodies, when combined with the radio-based wireless flash system.
(Hi-speed sync is possible for both E-TTL and manual flash exposure if a single 600EX-RT is used on-camera with previous EOS models, as well as during optical-based wireless E-TTL.)
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