Clarification
Arthur Morris has posted the reply he received from Chuck Westfall in regards to the f/8 issue with new Canon EOS-1D X.
“AF is unavailable on the EOS-1D X if the maximum aperture reported to the camera through the electronic lens mount is smaller than f/5.6. This is a lower specification than previous EOS-1 series DSLRs. On the plus side, consider the fact that with most f/4 lenses including the 400 DO, 500/4L IS and IS II, and 600/4L IS and IS II, you now have 41 cross-type AF points plus color and face detection, whereas you had no cross-type points and no color or face detection during AF with previous EOS-1 series DSLRs using the same lenses, not to mention a significantly wider AF coverage area from left to right.”
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