Digital Cameras and Combined Auto Focus | Hobbies

By DanFeildman
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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.

The focus can always be set manually by the photographer, but in most events the digital camera's automatic focus feature will prove much faster and simpler. Digital cameras incorporate many various algorithms and methods to automatically calculate the right focus for the situation. Passive and active auto focus are two such algorithms that used together create a combined automatic focus system delivering excellent automatic focus. Passive auto focus systems use software that runs on the digital camera built-in computer.

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 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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