Digital cameras include sophisticated mechanisms that can automatically figure out the perfect focus for the scene that you are photographing. However even the most sophisticated digital camera can be fooled by scenes that are hard to focus on. Here are a few such scenes and explanations.

There are three main ways to implement automatic focus:

Passive auto focus: This is practically an electronic version of the way in which photographers manually focus on objects. In this mode the digital camera defines areas in the photo on which it will try to focus. These are usually around the center of the photo. The camera captures the picture in real time and analyzes the quality of the focus in these areas. The camera moves its optical components (lenses) back and forth in a search for the optimal focus quality position. This in many ways is similar to the way photographers focus on objects ? they try to move the lens in one direction, if the focus gets better they move the lens a bit more, if the focus gets worse they move the lens to the other direction. The process continues until the best focus is achieved.

Active auto focus: Active auto focus is a more direct implementation of auto focusing. In theory if you knew the exact optical characteristics of the camera and its lens and the exact distance to the objects that you are trying to focus on ? you could calculate the exact position of the lens that will result in a prefect focus. Active focus systems measure the distance to the objects in the picture by transmitting an invisible light beam and measuring the time it takes for the beam to bounce back from the objecst. The measured distance is used to calculate the position to which the lens should be moved in order to achieve the best focus.

Combined auto focus: These systems are a combination of passive and active focusing. High end digital cameras support both passive and active methods. In combined mode the camera either chooses which system will work best for any given scene (for example in dark scenes passive auto focusing is not practical) or it uses both systems together (for example using the active system to bring the lens close to the perfect focus and then the passive system to fine tune that focus position.

In some scenes auto focus systems can fail. It is important to understand what these scenes are and it is even more important to understand why the auto focus system fails in these scenes. Photographers that understand how auto focus system works and why it fails can easily identify scenes in which the digital camera would not be able to automatically focus. In such scenes the photographer can use techniques such as manual focusing or focusing on another object in the same distance and panning the camera (while the focus is locked).

Following are a few scenes that will fail the auto focus system and the reasons why:

Low light or dark scenes: Passive auto focus systems need to electronically see the objects in the scene in order to calculate the quality of the focus and find he best focus position. In low light or dark scenes such systems can not work since they are not able to see the objects and to calculate the focus. Some digital cameras solve this problem by shooting a series of flashes toward the objects and having the passive auto focus capture the image and try to find the perfect focus. This solution is very limited. It can only work with objects that are within focus range. In addition such aggressive flash usage is not practical in many scenes for example if you are trying to quietly take a photo of an animal. In low light or dark scenes the usage of active systems is very effective since they do not need ambient light in order to work.

Active systems can fail too. For example they can fail when taking photos of objects that absorb the infrared energy that is used by the camera in order to measure the distance to the objects. They can also fail in scenes with objects that emit infrared energy such as candles or other heat sources. This emitted energy can cause the digital camera active system to measure the wrong distance.

Low contrast objects: Objects such as white walls or blue skies are low contrast objects. Passive auto focus systems rely on the fact that the focus quality significantly changes in relation to the optical components position. This is only true for high contrast objects. Low contrast objects are harder for the camera to focus on since the focus quality does not change much and it is almost impossible for the camera to find a definite best focus position. This can be solved by using an active system that can simply measure the distance to the object or by focusing on another high contrast object in the same distance and then panning to the low contrast object while keeping the focus locked.