To get sharp pictures, aim your camera to superimpose this target on the main subject. The obvious problem is that you won't always want to put your main subject in the center of the frame. To circumvent this, most auto-focus cameras have a focus lock. With it, you focus on your subject in the center of the viewfinder and then lock the focus. Once you lock the focus, you can recompose the scene.
Autofocus cameras focus on whatever is in the center of the viewfinder.
The background in this picture is sharp, the subject is unsharp. Charlie Fisher Many autofocus cameras have a focus lock. Depressing the focus lock will lock the focus of the camera. Using this method, the 2 subjects are now in focus. With most auto-focus cameras, partially depressing the shutter button activates the focus.