Note: This covers Nikon cameras, but many Canon cameras also have this option.
On just about any Nikon DSLR, you can set the camera to autofocus from a button on the rear of the camera, and make the shutter release only fire the shutter. This is a useful way to take an extra level of control over the camera.
Normally, the shutter release does double duty. A half-press engages autofocus, while a full press fires the shutter. In many cases, this works just fine, especially if you've grown up with autofocus cameras.
Separating autofocus control from the shutter release has several advantages, however.
If you grew up on manual focus cameras, locking the shutter release to just that one function may be more familiar. After all, manual focus cameras certainly have focus control separate from the shutter release!
By using a separate control for autofocus, you can maintain a high degree of control over when the camera engages autofocus. For focus-and-recompose situations, this is invaluable. Simply set focus with the rear button, release it, move the camera as needed, click the shutter.
