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Thanks for the additional information.
I understand, I think, the concept that multiple points add to the precision, I just don't understand how. The only time I really have an issue with focus is with birds at a distance; I have no idea how best to set up the AF to help track darting birds, nor do I understand how additional points somehow help to keep a little, moving object in focus (i.e., how does the AF system know that that little birdie is what I'm trying to focus on, rather than the background?). Maybe I'll get it after reading the...Show more →
If you only use 1 point during AI servo, then you have to keep the AF "over" the bird while it is flitting and swerving. Since that is tough to do, they allow you to make the camera more flexible. When you "expand" the AF points, the camera uses the one selected point to achieve initial focus - say at a distance of 60 feet. Then if the bird moves out of the area covered by that point, the camera software analyzes if any of the points surrounding the selected point have something in focus at 60 feet or so (since the object now covered by the initial point is the tree 120 feet in the background (again an example). If the camera does find something in focus at roughly 60 ft, then it assumes the bird has moved to that point and thus it uses the adjacent point instead of the selected point. In the case of the 1dx it also uses color to track if the object it thinks is the bird is not (i.e. if the initial lock was a white bird and the other point found green it is ignored).
Obviously this is a simplification, as many factors come into play when there are many objects at roughly the 60ft distance, and also the fact that the bird may be moving toward or away from you. That is why tracking a slow bird that moves in a straight line across a clear sky is easiest to track - a small bird in a marsh with reeds in the foreground is much harder, as are athletes running erratically on a crowded field. To help with those tough situations, the camera settings set by the user help the camera to more correctly chose the subject from the noise.