A quick post to describe my experience with the new auto focus (AF) system on the Canon 5D Mk III. Bottom line up front, dramatic performance increase when compared with the 5D Mk II and equal to the 1D Mk IV.
I spent a few days shooting with the Mk III and putting the AF system through some real world action shooting. You can easily read all of the technical specifications, beeps, and squeaks on many other blogs, I will give you my simple man's take on the AF performance.
The Mk III is a leap forward for the 5D series in terms of AF. There are numerous modes and configuration options but you will find the best performance in demanding action situations when you select the Single Point AF mode. In this mode you choose the appropriate AF point, engage AI servo with proper back button focus techniques and track your subject. Although this mode does not engage some of the advanced AF logic and new tracking "assist" features, it will deliver the best performance.
With that said, using a single AF point is not the simplest way to track and focus on a moving subject but, with practice, it is hands down the best. As always, we do not shoot in a vacuum and the specific shooting situation may require or force you to another mode. Depending upon the circumstances and variables you may get excellent performance with those as well.
I consider the 5D Mk III's AF performance to be equal to that of the 1D Mk IV and clearly superior to the 5D Mk II's and 7D's.
I look at action AF performance looking at one main variable (assuming a relatively consistent subject path), track crossing angle or TCA. TCA is the angular difference between where the camera is pointing (in aviation terms, where the camera is pointing is your effective "track") and the track of your subject. TCA may be either high or low angle, from a shooters perspective low angle is when a subject is in front of your lens and tracking directly towards or away from you. High angle is when the subject tracks in a perpendicular manner across your shooting direction.
Varying TCAs, when combined with subject aspect and speed, generates the multitude of tracking conditions necessary to get a good feel for what a camera can do. I will do a more detailed post with images and some diagrams regarding these topics soon.
For reference, the image at the top of this post is a 90 degree TCA at a high rate of speed. The second image is low TCA.