Digital Cameras and Combined Auto Focus

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.

It takes a combination of many optical parameters being set perfectly to create a high quality sharp and crisp photograph. One of the most vital optical parameters is focus. A photograph with objects out of focus appears blurry and lacks details and clarity. When objects are in focus the result is both 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 performs such image processing algorithms to establish a Focus Level number. This equation is much too complex to address here. Let's just say that the camera can calculate how good the focus is and find the best focus for the scenario. In some scenes the passive auto focus is limited or not serviceable at all. It does not work well in reduced 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.

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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