â€œAF 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.â€
via [NL] & [BPN]
I look at the Canon lens line-up and the only problem I see is when an extender is used. No Canon lens has a maximum aperture smaller than f5.6. Last century, I bought the 1.4x and 2x FD extenders and found the rendered IQ was lousy with all the recommended lenses. I used them only when absolutely needed, and never with pleasure. Although the EF extenders are supposedly better, I wouldn't use them; they're still a stopgap tool to be used when nothing else will work.Besides, I can always march 5 miles out on the tundra with this or this. Instead of extenders, why not utilize an APS-C sensor camera? The 800mm f5.6 lens becomes a 1280mm f5.6 lens with zero loss in IQ.
The MTF charts of the new super teles paired with the version III 2x and 1.4x extenders generally show little to no image degradation.
Also, although I understand your point about gaining more reach with an APS-C, it's just not true that using a 1.6x focal multiplier sensor equates to zero loss of IQ when compared to a 35mm full frame or aps-H sensor.
Shooting action at f8? No thanks.