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Question:

How does a camera's auto-focus work?

How does the camera know to focus on subject A when there are sorts of objects in its field of vision?


Best Answer - Chosen by Asker: What is Autofocus?
Autofocus (AF) really could be called power-focus, as it often uses a computer to run a miniature motor that focuses the lens for you. Focusing is the moving of the lens in and out until the sharpest possible image of the subject is projected onto the film. Depending on the distance of the subject from the camera, the lens has to be a certain distance from the film to form a clear image.
In most modern cameras, autofocus is one of a suite of automatic features that work together to make picture-taking as easy as possible. These features include:
Automatic film advance
Automatic flash
Automatic exposure

There are two types of autofocus systems: active and passive. Some cameras may have a combination of both types, depending on the price of the camera. In general, less expensive point-and-shoot cameras use an active system, while more expensive SLR (single-lens reflex) cameras with interchangeable lenses use the passive system.

For more info...

I. Active Autofocus
In 1986, the Polaroid Corporation used a form of sound navigation ranging (SONAR), like a submarine uses underwater, to bounce a sound wave off the subject. The Polaroid camera used an ultra-high-frequency sound emitter and then listened for the echo (see How Radar Works for details). The Polaroid Spectra and later SX-70 models computed the amount of time it took for the reflected ultrasonic sound wave to reach the camera and then adjusted the lens position accordingly. This use of sound has its limitations -- for example, if you try taking a picture from inside a tour bus with the windows closed, the sound waves will bounce off of the window instead of the subject and so focus the lens incorrectly.
This Polaroid system is a classic active system. It is called "active" because the camera emits something (in this case, sound waves) in order to detect the distance of the subject from the camera.

Active autofocus on today's cameras uses an infrared signal instead of sound waves, and is great for subjects within 20 feet (6 m) or so of the camera. Infrared systems use a variety of techniques to judge the distance. Typical systems might use: 1. Triangulation 2. Time
3. Amount of infrared light reflected from the subject

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II. Passive Autofocus
Passive autofocus, commonly found on single-lens reflex (SLR) autofocus cameras, determines the distance to the subject by computer analysis of the image itself. The camera actually looks at the scene and drives the lens back and forth searching for the best focus.
A typical autofocus sensor is a charge-coupled device (CCD) that provides input to algorithms that compute the contrast of the actual picture elements. The CCD is typically a single strip of 100 or 200 pixels. Light from the scene hits this strip and the microprocessor looks at the values from each pixel.
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