Let me preface this by saying that I have absolutely no clue as to how autofocus works.
I use an Elan 7N, with 7 AF points. This last weekend I was down at our aptly named Beachfront Park and I saw a hawk sitting right next to the waterfront foot/bike path. Just sitting there within 10 feet of cars. I was able to get about 10-20 feet from it before it flew down the path and parked itself again. It was totally unafraid.
I was using my camera's AI Servo AF mode which is like a cross between One Shot and Servo. It locks onto stationary objects but adjusts when it senses the lens-to-subject distance has changed.
I was framing vertically from below the bird with the proper AF point selected to get the bird more or less centered in the frame while focusing very carfully on the eye. When I saw that nice "catch-light" in the eye I took the shot. But several times during this "shoot" the AI Servo decided that the stationary bird was no longer in focus and adjusted to focus on some other thing (I don't know what) that was ever so slightly not the eye. The eye of the bird did not occupy the whole of the AF point and the bird did not move.
Now, the AF points I have (I'm not familiar with other cameras) are little rectangles.
My question: is there a specific part of this little rectangle that I should place my subject in for more accurate/reliable AF?
I'm just trying to figure out why the AF screwed up in this case...