The 5D Mark III comes with a new AF system that is, in terms of specification, very close to the flagship EOS 1D X. It comes with 61 points, 41 of which are cross-type points and, uniquely to this sensor, five of them are diagonally sensitive (for these double cross-type, imagine an X overlaid on a + shape).

When used with lenses with a maximum aperture of F5.6 or brighter, the 5D Mark III is unmatched in terms of the number of cross type points it offers (21). Use an F4 or brighter lens and the advantage becomes even greater, with the camera gaining another 20 cross-type points that are further out from the center of the frame (Nikon's system only features cross-type sensors near the middle of the frame). Fit an F2.8 or brighter lens and the five central double-cross-type sensors become available.

The 5D Mark III only loses out to the Nikon D4 and D800 when it comes to use with slower lenses or long lens/teleconverter combinations, in that its cross-type points can only be used with lenses that are F5.6 or brighter. Canon says there's a trade-off to be made and that its approach allows the sensor to be more accurate with the large aperture lenses it expects its customers to use, and allows the F4 cross-type sensors to be placed further towards the edge of the frame.

And, while the system doesn't have the 1D X's 100,000 pixel metering sensor, it still has a 63-point, color-aware metering sensor (a Foveon-esque two-layer affair), to help the camera track subjects.

F2.8 or brighter
F5.6 or brighter

The other key improvement is the simplification of the AF configuration. Previous high-end Canons have had very capable AF systems but have required fairly extensive training to correctly optimize them for the subject being shot. The 5D Mark III, like the 1D X, has a greatly simplified system for configuration, based on use-case presets.

AF configuration is complex compared to the 5D Mark II. But it's greatly simplified compared to the EOS-1D Mark IV, with a choice of 6 use-cases. The three parameters (tracking sensitivity, acceleration/deceleration tracking and willingness to switch AF points) can all be adjusted to more precisely tailor the presets to your favoured subject and shooting style.
The AF menu allows you to configure in detail precisely how you'd like the AF system to function, across fully five tabs of options.

Some of these have simply been ordered more coherently compared to where they were on the 5D Mark II, but there's a load of additional settings too, most of which are shared with the EOS-1D X.
You can choose which AF points you want the camera to use or allow you to select manually. The second setting - 'Only cross-type AF points' - takes into account the maximum aperture of the lens you're currently using.
AF microadjustment now allows you to program-in different settings for the two ends of a zoom. You can also enter an identification number for the specific the copy of the lens you're using.

We are no sports photographers but we can say that the simplified AF options are a great help when shooting moving subjects. We tested the EOS 5D Mark III's AF system with a 70-200mm F4L lens at an amateur soccer match, and even with our limited sports shooting skills got a very large proportion of usable shots. With the AF mode set to 'AI Servo' and the AF Case 4 'For subjects that accelerate or decelerate quickly' the camera locks onto a subject and tracks it reliably while shooting a burst.

The sample series below is representative for the kind of results we got. We locked focus on the player in the red shirt. The AF system tracks the subject but appears to get slightly confused when the subject is surrounded by other players. As a result in frames 5 and 6 the subject is very slightly soft, but even these shots would be usable on the web or at smaller formats. The AF also recovers quickly, with the subject in perfect focus again in the following shots.

Frame 1
Frame 2
Frame 3
Frame 4
Frame 5
Frame 6
Frame 7
Frame 8

In single AF mode the EOS 5D Mark III's AF performs very well, too, even in very low light, despite the lack of an AF-assist lamp. Rated down to EV-2, which is equivalent to moonlight, we've been constantly impressed by the ability of the 5D III's AF system to get solid focus in light where the 5D Mark II would have been completely unable to operate.

However, one thing to be aware of when shooting with wide apertures on a full-frame camera is the very little tolerance you have in terms of depth of field. When focusing on an image area outside the center of the frame it is advisable to move the AF point rather than using the center point and recompose. If you get consistent focus errors the camera's micro adjustment function can help, but in general it can still be a a good idea to focus bracket when shooting at longer focal lengths and wide apertures.

The camera's AF system works reliably and swiftly, even in low light. This picture was taken indoors at ISO 6400, 1/60 sec and F4.
And here's an even lower-light shot. Taken at ISO 25,600 at a shutter speed of 1/100sec and an aperture of f/5.6 the image in the viewfinder was barely bright enough for image composition yet the 5D III has captured a perfectly focussed, well-exposed image.

The contrast detect AF in Live View is still significantly slower than the standard phase-detect system. But this latest generation is now fast enough to make Live View a viable alternative for specialist applications such as macro or studio still life, or when the positioning of the camera makes it difficult to use the viewfinder.