How does auto-focus work?

All of us use it every time we click on the shutter button. Practically all modern cameras include some sort of an auto focus system. Thanks to the auto focus system we can enjoy an easier photography experience and can concentrate on composing the right photo and capturing the moment rather than on manually setting the focus.

Automatic focus though has its limitations. For example sometimes one might want to produce photos which are a bit fuzzy as an artistic expression. Also the auto focus implementation has its limitations and in some scenarios it might fail. One example is using a high end SLR camera with a passive auto focus system to take a picture of blue skies. In most cases the camera will move its motor back and forth and will eventually give up and fail to focus.

To better use the auto focus system it would help to understand high it actually works. Although implementations can vary we can divide them all into two categories: passive and active. Most pocket cameras use the cheaper passive method while high end professional cameras use either the active or a combination of both.

Passive auto focus:

Passive auto focus can be perceived as imitating how we set the focus manually. The camera defines one or more regions in the picture (usually they are marked as rectangles on the viewfinder or the LCD). The camera then analyzes the picture seen through those regions and calculates a Focus Level number. The camera then tries to move its lenses back and forth as it recalculates the Focus Level. The camera looks for a position where the Focus Level is the highest. For that point if the Focus Level is above a predefined threshold the camera would define this region of the photo as being in focus.

The Focus Level can be calculated in many ways. The common attribute of all calculations is figuring out how much Contrast is there in the photo. Although not in the scope of this article one way to calculate such a number is by running the photo through a high frequency filter