Canon has an AF configuration tool with 6 presets for photographing different types of motion. It is available on the Canon 1D X, 5D MIII, and there is some form of it on the 6D and 7D MII. These preset cases can help you get better focus when using AI Servo as the focusing mode. AI Servo continuously focus on a moving subject and these six presets contain parameters such as tracking sensitivity, Accelleration / decelleration tracking, and AF point auto swithching.
I've been experimenting with these cases for birds in flight. I used to use Case 1 and wasn't really happy with it. I tried Case 3 because I wanted faster (more sensative af tracking) and it seemed to work really well for this swan the was flying right toward me.
As you can see in the image above Case 3 uses +1 for Tracking sensitivity, which I found useful because I want to be on my subject as fast as possible and if my AF point falls off, that's OK if it can re-acquire focus very quickly. I've seen some bird photographers prefer to set this to -1 or 0 so the AF points don't get distracted by other objects and switch to focusing on something else, but I prefer to get on the subject quicker.
The accel. / decel. tracking is set to +1 which works well for subjects that tend to accelerate or decellerate quickly. Birds in flight may not change speeds quickly but again I'm looking for more sesitivity.
The AF point auto switching parameter isn't very important to me because I usually use a single point AF, sometimes with the 4 box expansion. Therefore, this function can be set low because I don't use it.
Here is a summary of all six cases:
Case 1 - Versatile multi purpose setting: 0,0,0. Adequate for larger birds in flight in open terrain, not flying right at you. The camera gets on the bird and stays with it if you stay on the bird. A good general setting.
Case 2 - Continue to track subjects, ignoring possible obstacles: -1,0,0. Example 1: When you’re locked onto a bird as it gets behind a floc, and you want to stay on the original bird. Example 2. A medium-sized bird in a thicket where you want the camera to ignore interfering branches as you follow a birds movement.
Case 3 - Instantly focus on subjects suddenly entering AF points: +1,0,0. You’re photographing a flock so it would make sense to pick up the closest birds as you usually want the closest thing to you to be in focus. Another scenarios is that you're on a bird and another nearby one flies in front of it, here it would make sense to switch focus to the near bird rather than the one now further away.
Case 4 - For subjects that accelerate or decelerate quickly: 0,+1,0. A large or medium bird coming right at you, coming in for a landing, taking off or buzzing by fairly quickly. An example would be where a bird leaps off the water, flies at you, and spreads it wings to land.
Case 5 - For erratic subjects moving quickly in any direction: 0,0,+1. Best for birds that change direction quickly such as leaping off a perch. Works well with 61-point AF expansion, and non-busy backgrounds. Best shot for this setting is to have the bird at a far left or right AF point in anticipation of it flying into the frame.
Case 6 - For subjects that change speed and move erratically: 0,+1,+1. Best for small birds that change direction quickly while in flight.
After all this experimenting, I decided to look up what Aurther Morris had to say on the subject, since he is a bird photography expert. It looks like I was off a little, he doesn't like Case 3 for birds except that he finds it the best one to customize. He says that every situation is different but for birds in flight you can't go wrong with this setup.
To create a custom Case, go to the first menu in AF, scroll down to the Case you want to over-write. Hit the rate button on the left side of the camera body and then press the Set button in the thumb wheel. Turn the dial to the left to -2 and hit Set. Scroll down to Accel./decel. and hit Set again. Turn the thumb wheel to 2 and hit Set. Do the same for AF point switching and set it to 2.
Click here for more information from Canon.
Click here for the Canon 5D MIII's AF Setting Guidebook
Also if anyone knows how Nikon handles focusing in AF-C, please post it in the comments.