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