I've found an oddity with the AF on the 5D MkIII. For a couple of weeks, I've been scratching my head over how I would select the outer points when using single point focus. After playing around this afternoon, I have found that if I select only the 41 most sensitive points, I can only select the central 21 points when using single point, single spot, or expanded, but if I switch to zone, I can select the outer zones. Switching to all 61 points or 9/14 points allows me to choose any point. Trying it out with other lenses (all in group A as opposed to group C for the 100mm macro), I can select any point I wish in any selection mode, including 61 points. I then tried another group C combination, the 300mm L IS iwth 1.4x extender. Again, I didn't have any problems with selecting any point I wished, so it isn't as simple as applying only to certain lens groups. From all of my lenses that are f/2.8 or wider maximimum aperture (I didn't try either the 24-105 or 17-40), it is only with the 100mm macro in 41 point mode where I can't select the outer 20 points and doesn't affect zone AF. I may have mised it, but I couldn't find any reference to the phenomenom in either the 5D MkIII manual or the 1D X AF guide. It isn't really a major issue, as most of the time, I would be using manual focus for macro work and I would be inclined to use the 135mm for the little portrait work I do.