It is really up to the person using the camera to determine if the subject is in focus. The camera merely assists you in making this decision. The two main causes of blurred pictures taken via autofocus cameras are:
- Mistakenly focusing on the background
- Moving the camera while pressing the shutter button
Your eye has a fast autofocus! Try this simple experiment: Hold your hand up near your face and focus on it, and then quickly look at something past your hand in the distance. The distant item will be clear, and your hand will not be as clear. Look back at your hand. It will be clear, while out of the corner of your eye the same distant item will not be as clear. Your camera is not nearly this quick or this precise, so you often have to help it.