One of the five things you need to know how to set correctly to make a well exposed and sharp shot in any given shooting conditions is focus mode. Fortunately, focus mode is by far the easiest of those five settings to set as there are only two choices (disregarding manual focus mode):
- AF-S (One Shot - Canon) for still subjects
- AF-C (Ai Servo - Canon) for moving subjects
That is all there is to choosing which focus mode to use. Is the subject moving or not? In the above example the chainlocked bike certainly is not moving so I used AF-S to shoot it. In the other example the triathlon cyclist was definitely moving, and fast, so I used AF-C.
Most DSLRs offer a third, and useless, kind of auto auto-focus setting that you should just pretend does not even exist!
So there is no reason to ever make a focus mode mistake if you can just remember AF-S (think "S" for still subjects") and AF-C (think "C" for continuous, moving subjects). No matter what you make a photograph of, moving or still, now you know which focus mode to use!