What is behind the auto focus(AF)

Auto focus is a very useful and typical function in current digital cameras and cellphone cameras. As I will work on some code about the Android autofocus, I read some martials from Wikipedia and this.

First, we define what is AF. A camera`s autofocus system intelligently adjusts the camera lens to obtain focus on the subject, and can mean the difference between a sharp photo and a missed opportunity. The process of autofocusing generally works as followed:

1) An autofocus processor(AFP) makes a small change in the focusing distance;

2) AFP reads the AF sensor to assess whether and by how much focus has improved;

3) Using the information form (2), the AFP sets the lens to a new focusing distance;

4) The AFP may iteratively repeat steps 2-3 until satisfactory focus has been achieved.

The entire process is usually completed within a fraction of a second. For difficult subjects, the camera may fail to achieve satisfactory focus and will give up on repeating the above sequence, resulting in failed autofocus.