Art Of Photography

Objects in good focus in a digital photo is a very basic requirement for high quality photography. There are two different ways to focus on such objects: manual or automatic. There are a few automatic focus methods - combined auto focus systems use more than one such method.

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.

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.

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.