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 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 this is based on the fact that high contrast is associated with high frequencies.
Active auto focus works by measuring the distance between the camera and the object in the picture. Technically if you knew the exact distance to the object you are taking a picture of you could
setthe lens to the exact focus position. The active focus system shoots a beam of invisible light, usually infrared, at the object at the center of the picture and measures the distance to that
object.Based on that distance the focus is set.
Some high end cameras combine both systems. The camera will pick the right system for the specific scenario or will cross check and use both at the same time. The photographer can also decide
manually to use one of the two options. For example when shooting blue skies the camera can try to use the active system and measure the distance. Since the distance is infinite the camera can
set the focus and skip the passive focus. In other cases when the distance is not infinite the camera can use the active system to put the lens in approximately the right position and then use
the passive system for fine tuning. In dark scenarios the camera can opt to use the active system since the passive one will not work.
