How Does A Digital Camera Combined Auto Focus Work

By Dan Feildman

Good focus of objects is a key element in producing a high quality digital photograph. You can achieve proper focus by two different methods, manual or automatic. We will concentrate on one method in particular that falls into the automatic focus category: combined auto focus.

It takes a combination of many optical parameters being set perfectly to create a high quality sharp and crisp photograph. One of the most vital optical parameters is focus. A photograph with objects out of focus appears blurry and lacks details and clarity. When objects are in focus the result is both sharp and crisp.

While the photographer can manually set the focus, most of the time the digital camera's automatic focus is much faster and easier. There are many different methods and algorithms to automatically establish the right focus for the situation that a digital camera uses. Passive and active auto focus are two of these algorithms. These two algorithms in particular are used in one type of combined auto focus system to provide excellent automatic focus.

Active auto focus systems use distance sensors measuring the distance from the camera to the objects in the scene. Typically the camera measures the distance to the object or objects at the center of the photo. Once determining that distance, the camera can then set the camera lenses to attain good focus. The ability to work in total darkness is one of the biggest advantages of active auto focus. This feature does have its challenges in unique situations. Active focus can fail to work when objects that emit certain energies (like infrared) are photographed or when surfaces in the scene absorb the energy used to measure the distance.

Passive auto focus systems use software that runs on the digital camera built-in computer. The camera executes such image processing algorithms to determine a Focus Level number. The exact way in which such a number is calculated is out of the scope of this article. Suffice is to say that the camera uses some image processing algorithms with which it can calculate how good the focus is. Using these algorithms the camera can find the best focus for the scene. In some scenes the passive auto focus is limited or not functional at all. It does not work well in poor lighting conditions and does not work at all in dark scenes. It is also hard to focus on low contrast objects such as walls or solid surfaces.

Cameras that have the combined auto focus systems are able to choose the right system for the circumstances or cross check by running both systems at the same time. The two options can also be determined manually by the photographer. An example of the camera knowing best is when photographing blue skies. The camera can attempt to use the active system and measure the distance. Since the distance is immeasurable, the camera can set the focus and skip the passive focus. In situations where the distance can be measured, the camera can use the active system to put the lens in the approximate position and then switch to the passive system for fine tuning. In a dark scene the camera would choose to use the active system as the passive system would not function.

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