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.
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 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 are equipped with combined auto focus systems pick the right system for the specific scene or cross check by using both systems at the same time. The photographer can also manually decide 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 system will not work.
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