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BY ZIV HAPARNAS

High quality sharp and crisp digital photos are a result of many optical parameters that need to be set right. One of the most important optical parameters is focus. When objects in a photograph are out of focus, they look blurry and are missing details and clarity. When objects are in focus, they look sharp and crisp.

While focus can be set manually by the photographer, in most cases using the digital camera’s automatic focus feature is much easier and faster. There are many different algorithms and methods that digital cameras use in order to automatically determine the right focus for a specific scenario. One of those methods is knows as active autofocus.
 

Active Autofocus

Setting the digital camera focus to its right position can be easy if you know the exact distance between the camera and the object or objects that you are trying to focus on. If you know the lenses that you are using, the aperture and all other optical settings, then simply by knowing the distance to the object or objects in the digital photo the exact focus can be calculated and set.

This is exactly how active autofocus works. Digital cameras that are equipped with an active autofocus system use distance sensors that measure the distance from the camera to the objects in the scene. Usually the camera measures the distance to the object or objects around the center of the photo.

There are a few techniques to measure the distance. One of them is by using an ultrasonic sensor. Such a sensor transmits an ultrasonic signal toward the object. When the signal hits the object it bounces back, and some of its bounced energy is received back by the digital camera sensors. The digital camera measures the time it takes the signal to return, and since the speed of the ultrasonic signal is known, the distance can be calculated. Other methods use infrared signals. Once the camera has determined the distance to the object, it can set the focus by calculating the exact position of the lens and then moving the lens to the determined distance setting.

Active autofocus has the advantage being able to work in complete darkness. Since the camera does not rely on measurements from a captured image, the camera sensors can calculate the distance to the object in complete darkness. This means that in focus high quality digital photos can be taken in scenarios where normal focusing otherwise would have been impossible.

Since the active autofocus method requires extra sensors, it is more expensive to implement and is usually found in higher end digital SLR cameras.

Active autofocus can sometimes fail. The reasons can vary: Some objects tend to absorb the transmitted signal energy instead of bouncing it back, while other objects actually radiate similar signals (for example, candles radiate infrared energy) and can confuse the digital camera sensors. When the autofocus fails, you can either try to focus on other objects of the same distance from the digital camera, lock the focus and pan back to the original object you want to capture, or you can revert to old fashioned manual focus.

Ziv Haparnas is a technology veteran and writes about practical technology and science issues. He write for printRates, a site dedicated to photo printing, where you can find more information about photo album printing and photography in general.