Spot AF mode:

You can manually choose any single AF point, but now actually reduce the size of that AF point. This allows you to read an even smaller area of the subject, and focus even more precisely on one particular element in a scene — such as the nearest eye in a portrait, or a precise part of a flower in a macro photograph.

When Spot AF is active, a small secondary box appears inside of the manually-chosen AF point, so you see a “box within a box” in the viewfinder. Like with Manual AF point selection, you can freely choose any of the 19 AF points for Spot AF, whether in the center or off-center. With Spot AF, like Manual AF point selection, you’re only using ONE AF point at a time.

A couple of important notes about Spot AF: While the AF point size is definitely reduced compared to ordinary Manual AF point selection, it’s not the tiny square you see in the finder. The actual area being sampled by the AF sensor is larger than that inner box, so allow for that when composing and shooting. Also, dual-axis, cross-type sensitivity remains at all AF points if you select Spot AF mode, so there’s no loss of effectiveness there.

The benefit of Spot AF — its ability to let you pin-point focus upon a tiny area of a scene — has obvious potential benefits and applications, and we’re sure advanced users and pros will come to appreciate this new feature. But this can have a possible downside, too. By forcing AF upon only a tiny area of your subject, if that happens to fall upon a part of your subject that’s relatively plain, solid and lacking detail, the AF system may have trouble focusing upon it. This can be a real problem with fast moving subjects, especially if they’re moving erratically. At times like these, actually having a larger area can be a more effective way to shoot.

For a more in depth information on the EOS 7D Auto Focus System, please click on the following link: http://www.learn.usa.canon.com/resources/articles/2011/whats_news_eos7d_article.shtml