I used this camera to do a graduation outside and a dance recital indoors. I used it with a 70-200 mkII IS with 2X III extender at F8 and the live view DPAF had to constantly be babysat. if someone else walked by on the stage it would grab their face and they would run away with the focus. During the dance recital I only used the 70-200 and there was 1 girl it absolutely would not hold focus on simply bc she had a dress on that was close to the color of the stage background or at least thats what I believe. It searched on her nonstop in all 3 scenes she was in. I had someone else shooting for me with a C100 mk2 and it had absolutely no issues with focus using the 100-400 mk2. It rarely went out of focus. Keeping focus 99% of the time even the moment the lights went up during a scene change. Id say the 1Dx mk2 kept focus 80% of the time but when the lights would go from black to bright it would hunt like crazy trying to focus for maybe 3-5 seconds. It would also randomly lose focus even on solo performances for a second or 2. The conditions at the graduation were ISO 100 1/125 F8-F11. At the dance recital ISO 1600-4000 F5.6 1/30. Im just not understanding why the C100mk 2 has such a more efficient focusing system. Something about them doesnt seem likely that they are really the same.