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I have an EOS 7D an I would appreciate any input regarding the best auto focus mode to use to capture birds in flight. Using a 500 mm lens and a gimbal head I tried Single Point, but found it too difficult to keep the point on the bird. I tried AF Point Expansion, with a little better luck, but still too many shots out of focus. While I understand that part of the problem may be "operator error," at the end of the day, I wondered whether I should have used Zone Auto Focus (or some other mode)? Thanks.
A couple things. You are on to something very important with your "operator error" comment. Don't feel bad. Photographing birds in flight (or any similar fast-moving object) is more about practice and technique than about gear... though gear and right setting choices are also important.
The hint for me was your "too difficult to keep the point on the bird comment." It is, indeed, difficult. At first, it is nearly impossible. However, with practice you can learn to keep that AF point where you want it. With more practice you can start to be conscious of and even control the position of the bird within the frame, and eventually you can even become conscious of and able to control the placement of the bird relative to background elements. On some shots...
So, a lot of practice is required. If I can make an analogy, if you were hunting and missing the target, the solution wouldn't be a better sight or a longer barrel... it would lie elsewhere, right? That same "elsewhere" applies here, too.
I think that as you do more of this you may also discover that it isn't so much the right setting as it is the best setting for the particular circumstances. For example, if there is any sort of complex pattern behind the bird, you will almost certainly be better off with a single focus point - otherwise a single point over the background can cause the camera to AF the background instead of the bird. On the other hand, shooting against the sky might work just find with multiple points - there is nothing for the camera to focus on other than the bird. In a few cases - when the birds follow a known trajectory, for example - it is even possible that pre-focusing and turning of AF can work.
The simple answer for me is that I'm most likely to use AI servo with a single AF point. I put the camera in burst mode most often, but most of the time I don't hold the shutter down long enough to actually burst.