[hollis_f]

Hi Clive,

With One-Shot the focus locks and then doesn't change as long as you've got the shutter half-depressed. That allows you to focus and recompose. It's also useful if you've got interfering objects that may pass between you and the subject (wildlife in trees or long grass is a good example). In the other modes the camera may well decide to try to focus on the closer object as it moves near your subject. In One-Shot the camera will only take a picture when it's locked the focus on something.

AI-Servo is great for keeping focus on a moving subject - as long as it doesn't move behind something else. But you have to bear in mind that the shot will be taken whether or not the camera is focussed.

AI-Focus sounds great. But it's my experience that it seems to combine the disadvantages of the other two modes, rather than their advantages. With a static subject it'll switch to Servo mode just when you don't want it to. And it's very often too slow to cope with a moving object.

So I tend to use One-Shot or AI-Servo, depending on how I think my subject will be moving.