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Actually, I have a question for clarification. Here is my understanding:
Custom controls, go to AF-ON, scroll over to AF-OFF and press set.
If my drive is set to servo, I hold down the AF-ON back button to continuously focus (servo). When I want a single shot, I release the back button and depress the shutter I usual. Is this correct?
No, if you press Set for AF-OFF, it means you've just reversed the function of the AF-ON button. When you press it, the camera will stop AF.
You want to set the AF-ON button to metering and AF start, and then set the shutter button to metering start only. That removes AF from the shutter button and places it only on AF-ON.
As for leaving the camera in Servo and using the AF-ON button to simulate One Shot, I think Form is correct - it won't be as reliable as One Shot when focusing on difficult subjects. This has certainly been my experience with the xxD family of cameras, but my 7D is better, and I have always heard that the 1-Series are better still. So with the 5DIII's advanced AF, I would expect this to be less of an issue, and Servo probably works pretty well on non-moving objects.