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.
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 focus can be manually set by the photographer in most cases using the digital camera's automatic focus feature is much easier and faster. There are many different algorithms and methods that digital cameras use in order to automatically determine the right focus for a specific scenario. Such algorithms include passive and active auto focus. One type of combined automatic focus system uses these two algorithms to delivery superior automatic focus.
Active auto focus systems use distance sensors that measure the distance from the camera to the objects in the scene. Usually the camera measures the distance to the object or objects around the center of the photo. By knowing that distance the camera can then set the camera lenses to achieve good focus. One of active auto focus biggest advantages is that it can work in complete darkness. On the other hand active focus can fail in problematic scenarios such as when objects 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.
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