How does a digital camera active auto focus work

How does a digital camera active auto focus work
by Ziv Haparnas


In focus objects 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 one of them is known as active auto focus.
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 digital photo 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 auto focus.
Active auto focus
Setting the digital camera focus to its right position can be easy if only you knew 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 the other optical settings then by 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 focus works. Digital cameras that are equipped with an active auto focus 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 took the signal to reach back the camera and since the speed of such an ultrasonic signal is known the distance can be calculated. Other methods use infrared signals. Once the camera determined the distance to the objects the focus can be set by calculating the exact position of the lenses and by moving the lenses to that position.
Active auto focus has the advantage of working in complete darkness. Since the camera does not rely on measurements done on the captured photo the camera sensors can calculate the distance to the object in complete darkness and in focus high quality digital photos can be taken in scenarios where otherwise focus would have been impossible.
Since the active auto focus method requires extra sensors it is more expensive to implement and is usually found in higher end digital SLR cameras. Active auto focus 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 auto focus fails you can either try to focus on other objects in the same distance from the digital camera, lock the focus and pan back to the original objects you wanted to capture or you can revert to old fashion manual focus.
About the Author
Information about photography and photo prints is on printrates.com - your home for digital prints This article can be reprinted as long as the resource box including the backlink is included. Ziv Haparnas writes about practical technology and science issues.