The autofocus range of the 5DIII is from EV -2 to EV +18, this must be based on the amount of the light entering the lens and the sensitivity of the autofocus sensors.
If it can autofocus at EV -2 at f5.6, can someone explain why it can't autofocus at f8 at EV -1, f11 at EV 0 or f16 at EV +1.
Why the f5.6 restriction, it is so limiting particularly when using extenders.
I have yet to see someone using a f2.8 telephoto, where this wouldn't be an issue.
In the UK the 400mm f2.8 and 500mm f4 cost around £10,000 (16,000 USD).
I have the excellent 400mm f5.6L, but soon as I put on a 1.4x, I have manual focus, and the lens doesn't have IS, so it difficult to use.
I think autofocusing down to f16 lenses should be standard.