You may not always want your subject in the middle of the picture. If your subject is not in the center of the picture, you need to use the “Focus lock” feature of your camera.  Most auto-focus cameras focus on whatever is in 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.

You can use focus lock in three easy steps.

First, center the subject and press and hold the shutter button halfway down. Usually a green light will appear].

Second, reposition your camera (while still holding the shutter button) so the subject is away from the center.

Third, finish by pressing the shutter button all the way down to take the picture.

Don’t forget to tell your subject to say, “cheese”!