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.
Some cameras have a separate button that locks the focus until a picture has been taken.
If the distance does change, the subject will likely be out of focus. Refocusing is a simple matter of re-centering your subject and again partially pressing the shutter release button. The single-shot mode is better for photographing stationary subjects. The camera will not fire until the camera has confirmed sharp focus. The servo mode works well for photographing moving subjects-a girl on a bicycle, for instance.