How well does the A99 work for sports photography

When properly configured, having an AF cluster spread out can give you precise control over what you're focusing on, and help keep the subject in focus as it moves around. With dynamic area AF (Canon calls this AF area expansion), you have a single point selected. That lets you easily pick what subject to focus on - just put the selected point on it.

However, a number of surrounding points can kick in if the camera isn't able to keep tracking with your selected point. That can happen if the selected "main" point ended up over a low contrast, hard to focus on area, or if imperfect tracking by the photographer briefly puts the "main" point off the subject but some of the expansion/dynamic area points are still on.

Since fast moving, unpredictable subjects are difficult to track perfectly, in practice letting the camera shift to a specified number of other points as needed can work very well.

For example, take the D600 in the rather extreme 39-point dynamic area mode below. I have the center point selected, and only the center point is visible to keep the finder uncluttered, but all 39 points can "assist" if the selected center point can't track for some reason. Despite my best efforts, the center point is over a low contrast area that's rather hard to focus on. The camera picked another area (with high red/white contrast) that it can more easily use to track the subject. And making sure distance information is reasonable given previous subject behavior (like what the a99 does with AF-D?) prevents the camera from focusing on the background.



So it's like a safety net in case the photographer isn't able to present the AF system with perfect information all the time, which happens a lot with fast, unpredictable subjects. Hopefully this makes sense?