By default, most SLRs autofocus when you half-press the shutter button. If you want to recompose before taking the picture, you have to keep the button held down while you recompose. This can be awkward, which is why many photographers choose to do things differently by remapping autofocus to a separate, dedicated button. I first learned about this technique when I was shooting sports and have since taken it for granted. With the release of the 5D Mark II and it’s new AF-ON button, I thought some people might be wondering what it’s all about.
To get the benefits of the AF-ON button, you need to change Custom Function IV-1 to 2, which will disable the default autofocus behavior of the shutter button. Now the camera will only autofocus when you press the AF-ON button. Once you release the button, the focus will stay where you left it and you’re free to recompose at will. If the camera-to-subject distance isn’t changing, you might take many photos before refocusing again.
One of the things I really like about this setup is that I don’t need to worry about turning autofocus off when I’m composing a landscape image on a tripod. I usually focus such images manually so as to maximize depth of field. Since most of the lenses that I use feature full-time manual focusing, I don’t even need to move the switch on the lens out of the AF position.
Finally, if you want to track a moving subject, just set your AF mode to AI SERVO. Now you can hold down the AF-ON button to track your subject. In fact, I leave my camera set to AI SERVO all the time. For static subjects I just press the AF-ON briefly until I get a focus lock. Technically ONE SHOT is supposed to be more accurate for static subjects, but I haven’t had any problems. Besides, if I want critical focus on a static subject I’m more likely to check focus by eye or with live view.
If your camera doesn’t have a dedicated AF-ON button, most Canon SLRs allow you to set a custom function that moves AF activation to the * (exposure lock) button. Other manufacturers usually include similar functionality.
Update: One more thing. Maybe it goes without saying, but this technique works best if you’re using a single AF point.