| ality photograph is comprised of many elements, | | | | are photographed or when surfaces in the scene |
| one of which is objects being in good focus. | | | | absorb the energy used to measure the distance. |
| There are two different ways to focus on | | | | Passive auto focus systems use software that |
| objects: automatic or manually. There are a few | | | | runs on the digital camera built-in computer. The |
| automatic focus methods and combining more | | | | camera executes such image processing |
| than one of these is known as combined auto | | | | algorithms to determine a Focus Level number. |
| focus systems. | | | | The exact way in which such a number is |
| High quality sharp and crisp digital photos are a | | | | calculated is out of the scope of this article. |
| result of many optical parameters that need to | | | | Suffice is to say that the camera uses some |
| be set right. One of the most important optical | | | | image processing algorithms with which it can |
| parameters is focus. When objects in a digital | | | | calculate how good the focus is. Using these |
| photo are out of focus they look blurry and are | | | | algorithms the camera can find the best focus for |
| missing details and clarity. When objects are in | | | | the scene. In some scenes the passive auto |
| focus they look sharp and crisp. | | | | focus is limited or not functional at all. It does not |
| While the focus can be manually set by the | | | | work well in poor lighting conditions and does not |
| photographer in most cases using the digital | | | | work at all in dark scenes. It is also hard to focus |
| camera's automatic focus feature is much easier | | | | on low contrast objects such as walls or solid |
| and faster. There are many different algorithms | | | | surfaces. |
| and methods that digital cameras use in order to | | | | Cameras that have the combined auto focus |
| automatically determine the right focus for a | | | | systems are able to choose the right system for |
| specific scenario. Such algorithms include passive | | | | the circumstances or cross check by running both |
| and active auto focus. One type of combined | | | | systems at the same time. The two options can |
| automatic focus system uses these two | | | | also be determined manually by the photographer. |
| algorithms to delivery superior automatic focus. | | | | An example of the camera knowing best is when |
| Active auto focus systems use distance sensors | | | | photographing blue skies. The camera can |
| that measure the distance from the camera to | | | | attempt to use the active system and measure |
| the objects in the scene. Usually the camera | | | | the distance. Since the distance is immeasurable, |
| measures the distance to the object or objects | | | | the camera can set the focus and skip the |
| around the center of the photo. By knowing that | | | | passive focus. In situations where the distance |
| distance the camera can then set the camera | | | | can be measured, the camera can use the active |
| lenses to achieve good focus. One of active auto | | | | system to put the lens in the approximate |
| focus biggest advantages is that it can work in | | | | position and then switch to the passive system |
| complete darkness. On the other hand active | | | | for fine tuning. In a dark scene the camera would |
| focus can fail in problematic scenarios such as | | | | choose to use the active system as the passive |
| when objects emit certain energies (like infrared) | | | | system would not function. |