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High quality photograph is comprised of many elements, one of which is objects being in good focus. There are two different ways to focus on objects: automatic or manually. There are a few automatic focus methods and combining more than one of these is known as combined auto focus systems.

High quality sharp and crisp digital photos are a result of many optical parameters that need to be set right. One of the most important optical parameters is focus. When objects in a digital photo are out of focus they look blurry and are missing details and clarity. When objects are in focus they look 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.

Active auto focus systems use distance sensors that calculate the distance from the camera to the objects in the scene. Typically the camera measures the distance to the object or group of objects nearest the center of the photograph. In calculating this distance the camera then programs the lens to achieve the best focus. One of the biggest advantages to active auto focus is its ability to function in total darkness. Active focus can fail though in challenging situations such as when objects that produce certain energies (like infrared) are photographed or when surfaces in the scenario absorb the energy that is used to calculate the distance.

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.