AF-ON...

Those who have meet me, know that one of the first tips I offer to fellow photographers is to take AF control away from your shutter button. In this months 'Outdoor Photographer' George D. Lepp suggest the same thing, as he is George Lepp, I'll let his words tell you what to do:

AI Servo is a predictive focus feature found on Canon DSLRs; Nikon offers a similar capability called Continuous Focus. These modes are especially useful for wildlife/bird and sports photographers because they allow the photographer to “lock” onto a moving subject and keep it constantly in focus by predicting its next location. The photographer can concentrate on keeping the subject in the frame while the camera keeps the subject in focus. As I’ve written before, predictive autofocus, introduced in the mid-’80s, was one of the great game-changer advances in photography.

When I’m photographing wildlife in the field (or on my grandkids’ soccer field), I want to be able to shoot both moving and still subjects without changing my camera settings. This is accomplished not so much with autofocus modes, but rather by the camera’s custom functions. Straight out of the box, I program all of my Canon DSLRs to deactivate autofocus from the front shutter button and activate the autofocus (AF) button on the rear of the camera. From that point on, I’m using my thumb on the rear AF button to focus the camera and my index finger on the shutter button to capture, and because autofocus isn’t active on the shutter button, the focusing decision I make isn’t overridden by the camera as the image is captured.

With this setup, the AI Servo tracking is active only when you depress the rear autofocus button. Keep it depressed as you follow the action, and capture your images with the front shutter button. When you lift off the rear shutter button for a stationary shot (as for a single player on the field or a portrait), focus is locked at that location. You can reframe the image, then capture with the shutter release. When your thumb is off the rear AF button, you can manually focus with the lens’ focusing ring and override any previously set autofocus. AI Servo mode is only activated when you depress the rear autofocus button, so leave it on all the time. With this setup, you have it all.

The full article is here