The 5D series finally gains a professional AF system with the 61-point AF upgrade, the same system as found on the flagship 1D X. The Mark III's AF has its own AF menu and six possible AF Area Selection Modes. AF Modes are the standard One Shot, AI Servo and AI Focus AF found on other Canon DSLR cameras.
The AF menu is used to fine-tune AI Servo AF but can be a little confusing. There are six different cases, which are varying combinations of tracking sensitivity, acceleration/deceleration tracking and AF point auto-switching.
Case 2 for example, will "continue to track the subject, ignoring possible obstacles", while Case 4 is "for subjects that accelerate and decelerate quickly." While we can see its applications for different sports, it seems tough to define the various use cases. For example, Case 5 is "for erratic subjects moving quickly in any direction" while Case 6 is "for subjects that change speed and mover erratically."
If these descriptions don't work for you, you can adjust each case's three parameters yourself. You can also simply use Case 1, which is a "versatile multi purpose setting" for "any moving subject."
AF Area Selection Modes has also been expanded with six options. Single-point Spot AF gives you a single point for precision focusing, while single-point AF selects one AF point for focusing. Sounds similar? Single-point Spot AF is essentially the same as Single-point AF, but the AF point covers an even smaller area inside the AF point (illustrated by a small square within the AF point) to focus. This is a precision method for when you need to be really precise with your focus point.
Then there are the two AF point expansion options. In the first AF point expansion mode, the AF point, as well as the four AF points above, below, left and right of it are used to focus. In the second AF point expansion mode, the AF point and the surrounding eight adjacent points are used to focus. Next, Zone AF divides the 61 AF points into nine zones for focusing, where all the points within a zone are used to achieve focus.
Together, these three AF Area Selection Modes are the most useful for manual AF area selection when shooting moving subjects. The last AF Area Selection Mode is 61-point automatic selection, where the camera will find the focus points itself. Set in tandem, the AF Area Selection Modes and AF Modes provide a wide variety of auto-focusing shooting styles. If these are one too many modes for you, you can even custom toggle some of them off.
Like on the Mark II, the joystick can be custom set to trigger AF area selection immediately, without having to press the AF point selection button first. The one area of weakness lies in switching modes on the fly using the top LCD panel. To choose between the six different modes you press the AF point selection button, and then press the M-Fn button to switch modes. The LCD panel is not helpful in this regard, as it'll simply display the same "SEL" title no matter which manual AF selection mode you switch to. To see which mode you're in, you have to either look through the viewfinder or press the Info button to have it displayed on the back LCD monitor. This is an area for future refinement.
We found that in general, the 5D Mark III showed a marked improvement in auto-focus speed and accuracy over the Mark II. The camera usually nails the right subject in automatic selection mode, and the expanded 61 AF points certainly play a big part. We did find that the camera will sometimes struggle in low-light, but will do well in most indoor environments. It will track reasonably fast moving subjects well, like people walking and running, but it will sometimes get confused with very fast subjects like gymnasts and tracking in/out of the speedy subject.
While Canon says the Mark III shares the same AF system with the 1D X, it's more than likely that the 1D X will out-perform the Mark III. The 1D X comes with three processors while the Mark III has one, and one of the three processors in the 1D X is dedicated to metering and AF alone. Still, with the Mark III's relatively fast six frames per second shooting speed, we find that it is quite possible to shoot sports with this camera using the right combination of AF Area Selection Modes and AF Modes.
The camera comes with a further option to fine-tune its shooting performance when shooting bursts. You can tell the camera to put priority on either focus or release on the first frame, and focus or speed on the second frame.
What this means is that the camera will either lock focus first and then shoot (priority on focus, first frame), or it will simply start shooting the moment you press the shutter even if focus hasn't been achieved (priority on release, first frame). Subsequently, the camera will either lock focus in-between each successive shot before shooting (priority on focus, second frame) which may slow down the maximum frames per second, or it will simply keep on shooting (priority on speed, second frame). Be default, the camera sets equal priority to both parameters.