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How Does A Digital Camera Combined Auto Focus Work

Jul 31, 2008
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.

There are many visual parameters that need to be set correctly to achieve high quality sharp and crisp digital photos. One such parameter that is vital is focus. When objects are in focus they look sharp and crisp. But should they not be in focus, the result is a blurry photograph lacking detail and clarity.

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.

Those cameras that are outfitted with combined auto focus systems have the ability to use the right system for the specific situation or cross check by using both systems at the same time. Of course, the photographer can also manually choose which option if they so desire. As an example, when shooting blue skies the camera can try to use the active system and calculate the distance. Since the distance is endless the camera can set the focus and skip the passive focus. In other cases when the distance is not endless the camera can use the active system to put the lens in about the right position and then use the passive system for fine tuning. In dark circumstances the camera can choose to use the active system given that the passive system will not work.
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