You'll find your answer in this...‘C.Fn III: Autofocus/Drive.’ Some of these settings are among the most important of all 7D settings when it comes to getting the best possible images. I have ‘C.Fn III 1 AI Servo tracking sensitivity’ set to ‘Slow.’ If I’m tracking a flying bird and it flies behind a tree, slow tracking sensitivity means the camera will not give up focus on the bird and grab focus on the tree. This setting makes the AF much less twitchy. I have ‘C.Fn III 6 Select AF area selec. mode’ enabled. If you then select ‘Register’ you can place check marks next to the types of AF you use. Once you have your check marks finished you select ‘Apply’ to finalize your selections. To me this is probably the most critical setting of all. I have both of the automatic focus point selection options unchecked (these are the first 2 choices on the menu). If you want your images to be critically sharp where you want them to be sharp, don’t rely on the camera to choose your focus point for you. Using these 2 focus options is akin to shooting in full auto mode in my opinion, and is the cause of many out of focus images being posted on the internet forums. I have the last 3 AF options enabled (’AF point expansion,’ ‘Single point AF,’ and ‘Spot AF.’ You can cycle through the active focus modes by pressing the top right button on the back of the camera and then repeatedly pressing the M-Fn button just behind the shutter release button. I use ‘AF point expansion’ and ‘Single point AF’ for birds in flight. ’Spot AF’ is a more precise version of ‘Single point AF;’ it’s like an AF point within an AF point. I use it for perched subjects when I want to get the eye sharp. When using Single point and Spot AF, I often move the focus point around the frame in an attempt to compose correctly through the viewfinder (if a duck is moving across the water from my right to my left, I will move my focus point above and to the left of center and try to place the focus point on the bird’s eye). The next setting I alter is ‘C.Fn III 12 Orientation linked AF point.’ I have it set to ‘Select different AF points.’ With this setting, I can select both an AF mode and an AF point for the camera to use depending on whether the camera is in a horizontal or vertical orientation; the camera senses its own position and makes the adjustments automatically.
This was taken from Doug Brown's article at http://www.deepgreenphotography.com/blog/?p=226