Depending upon the shooting conditions, it may take slightly longer for the EOS 5D Mark III and EOS-1D X digital SLR cameras to acquire focus when using the Speedlite’s AF Assist Beam, compared with that of the EOS 5D Mark II and EOS-1D Mark IV digital SLR cameras using the Speedlite’s AF Assist Beam.

We are developing a solution to enhance the focusing feature for the EOS 5D Mark III and EOS-1D X and plan to release a firmware update based on following estimated schedule:

EOS 5D Mark III: (estimated) Late April, 2013

EOS-1D X: (estimated) Late May, 2013

via Canon U.S.A. : Support & Drivers : Consumer & Home Office.