How Autofocus Cameras Work?

Over the course of many decades camera technology has progressed from non-moving methods of exposure through mechanical systems to fully electronic cameras that require little more than pushing a button. Getting a sharp photo from these cameras has always involved making sure the distance between the camera lens and the object is correct.
Early photographers had to move the entire camera because there was no focusing mechanism on the stationary equipment. As technology improved manufacturers were able to create lenses that were manually focused by turning the lens housing until the image appeared to be sharp. Now, even the most inexperienced photographer can be sure of getting a focused photograph most of the time. How can they be so sure?

All they have to do is use an autofocus camera.
Autofocus systems in modern cameras involve moving the lens with a small motor and the electrical power of the camera’s battery. In simple terms, the lens moves back and forth – in and out in relation to the film or recording surface inside the camera. When the sharp image is perceived by the computer chip in the camera the lens stops moving and you are ready to get your picture.

When people think of and use autofocus cameras they generally don’t pay much attention to this feature unless it doesn’t work properly. But there is a little more to know about autofocus than what we have already discussed. There are two types of autofocus systems in use. They are known by the common names of active and passive. Cameras may have a combined active/passive system, though most low-price cameras use the active system. More expensive cameras usually have passive autofocus technology.

Active autofocus methods began with the use of sound to determine whether the distance from the object was correct. More recently, manufacturers have used infrared technology to determine if the photo subject is at the correct distance. Technical manuals for this type of camera show that the signal might involve the amount of time for the signal to be returned, the light level reflected from the photo subject and/or triangulation. The camera sends out a light signal and judges the distance, time and reflected light to make sure distance is correct. This system can even work when the photo is taken in complete darkness.

A passive autofocus system found on more expensive cameras, such as digital, single-lens reflex cameras, works much differently. No sound or infrared signal is sent out. Instead the camera’s computer chip analyzes the object and moves the lens in and out until focus is achieved. Basically, light reflected from the photo subject contacts an imaging strip in the camera. This system is called “passive” because the camera doesn’t send out the sound or light signal. While this type of system is generally more accurate than the active system used on less-expensive cameras, it does require sufficient light and distinct contrast on the subject. In low-light situations, the camera may not be able to detect enough details on the photo subject to start the focusing process within the camera.

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