If your subject is not in the center of the picture, you need to lock the focus to create a sharp picture. Most auto-focus cameras focus on whatever is in the center of the picture. But to improve pictures, you will often want to move the subject away from the center of the picture. If you don't want a blurred picture, you'll need to first lock the focus with the subject in the middle and then recompose the picture so the subject is away from the middle.