It has been said that autofocus is Canons strong point and is why all the sports shooters went to Canon in the 1990's.
On unique and special occasions a different single point other than the center point may be selected. The exception that breaks the rule is sports/action photography. Selecting a single autofocus sensor is really the only way to have complete control over what is going on. Setting the camera to do automatic focus point selection is a lot like (and goes hand in hand with) using "green box mode". You are making your camera into a fully automatic point and shoot in hopes that it will help you grab unexpected and fleeting shots -- which may be well justified as well as a wise tactic in certain situations.
This makes you wonder if the whole business of multiple focus points is another numbers game like megapixels. Bigger numbers help sell cameras - and have some utility towards making better pictures.
The following diagram shows the whole situation:
A cross type sensor is one that can detect both horizontal or vertical lines. Vertical line detection is about twice as sensitive as horizontal line detection for cross type sensors. Sensors that are not cross type sensors are horizontal line sensors only.
Note that for the 1D, all the cross sensors work only for lenses f/2.8 or faster. With an f/4 lens, only the center point is a cross type, all the others are sensitive to horizontal lines only. With an f/5.6 lens, all sensors are horizontal line only. With an f/8 lens only the center point works, and it is only horizontal line sensitive.
When the 70-200 f/2.8 L lens is used with an extender, only the center point should be used (the combination is f/4), all other points will cause an error.
The 5D Mark II camera has 9 main points plus 6 assist points.
The points are laid out like this, note that only the center point is a cross point.
The assist points (also shown in the above) are like so:
The 20D has a 9 point system, which like the 5Dii has only a center cross point. The center point is only a cross type with f/2.8 or faster lenses. All the other points work with lenses f/5.6 or faster. There seems to be no mention of assist points with the 20D. (In short, the 5Dii has a little better autofocus system than the 20D, but not by much.)
Note that the autofocus points (all of them) give up entirely beyond f/5.6. This is not an issue for me with any lens combinations I use. I situation where it might be an issue would be if you used an f/4 supertelephoto with a 2x teleconverter. This combination would be f/8 and autofocus would not work. There was a firmware fix for the 1D Mark III to allow f/8 autofocus with these supertelephoto lens combinations.
The 5d2 has an option to allow you to display which autofocus points were actually used while you are reviewing an image. This is a good diagnostic tool when you are trying to understand why an image is not focused like you might want.
Newer cameras (such as the 7D, 5Diii, and 1Dx -- which I do not own) have a 61 point autofocus system. (with 41 cross type sensors). Among other things it allows selecting groups of points in "zones".
One shot mode is simple enough, and what I use almost all of the time. When you press whatever button you use to trigger focus, the camera focuses one time and is done. When the camera achieves focus, a red indicator in the viewfinder comes on and the camera will beep. I always turn the beep off. If the camera cannot achieve focus, the red indicator in the viewfinder will blink and it will not be possible to take a picture.
AI Servo is the smart mode that sports and action photographers use. As long as you hold the focus button down, the camera follows a moving subject with focus. It uses the center focus point to achieve the initial focus. You do not get a beep or a red viewfinder indicator.
The 1D Mark III camera has a number of custom functions that govern how this feature works and some long articles have been written about how to get them set up properly.
The 5D Mark II adds a mode it calls "AI Focus". I have to ask why this is needed on the 5D camera while it is not needed on a 1D body? What it claims to do is to change from "one shot" to "AI Servo" if the subject starts moving. It sounds like a sort of insurance policy mode.
What do you do when you are in "automatic" and the autofocus keeps making a choice you don't like? You wish you were in center point only mode, that is what you do.
The following article is quite instructive, and not just for 1D cameras.
On each of my cameras, the button on the uppermost back of the camera is labelled with what looks like a rectangle with a little plus symbol inside of it. This activates the focus point selection mode. We will call this the "focus point selection button. Once you press this, you have 6 seconds to use the dial, wheel, or joystick to select among focus points. When all the focus points light up (or in the case of the 1D, the outer "ring of fire" lights up), you are in automatic focus point selection mode.
The 5Dii has a great way to handle this via control function III-3. You set it for "multicontroller direct" and you can use the joystick (AKA: multicontroller) to jump between single autofocus sensors without having to push any other buttons. I pretty much don't use the joystick for anything, so dedicating it to control of autofocus points is great for me. The selected point blinks and it is selected. To get to automatic point selection, push the focus point selection button and all focus points blink once. This is wonderful. The camera has to be awake to do this (and it goes to sleep pretty quickly in 3 seconds or so).
The best way I have found so far with the 1D to get between automatic and single center point also involves the joystick. Press the focus point selection button and then press the center of the "joystick button". This will jump back and forth between automatic and center point only mode, which I think is great.
Each camera has a multitude of special functions that can be used to customize the way the autofocus system works, especially the 1D which was designed to please sports/action shooters.
Tom's Digital Photography Info / tom@mmto.org