By default, almost every camera arrives from the factory with two important functions assigned to one big conspicuous button: the shutter release. It’s conveniently located where the right index finger would rest when holding the camera comfortably in your hands. This button will engage autofocus when pressed halfway and fire the camera when fully depressed.
At first glance, it might seem like a more complicated way of doing things, but it’s not. After a minor adjustment period, it will soon feel as natural as the default set up, maybe more so. It’s just so easy; thumb focus, finger capture. There are many things we do in everyday life which involves using the thumb and index finger simultaneously. And some of these things are actually good.
As I said, I find this way of focusing most valuable when I’m doing wildlife photography but I use it for any scene where I’m using autofocus. Focusing with my thumb is now a grooved habit that has taken quite a bit of time of set in. The middle of an expensive Africa safari is not a good time to experiment with back button focus if you’ve never used it before. Under stressful situations (a lion stalking your safari vehicle with a setting sun over it’s left shoulder, as an example) you will defer to the familiar without even thinking and this shot of a lifetime will be out-of-focus because you assumed it was focusing with the shutter release button.