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Autofocus: Searching for the Sharpest Image
Manipulating all your camera’s controls while composing a shot can be overwhelming. Read on to learn how autofocus takes some of that hard work off your plate.
The human eye is among nature’s most complex objects, so it’s no surprise that a camera’s autofocus function should require a lot of little moving parts—and still not be able to focus as rapidly as an eye. The basic components of all autofocus systems are the same: based on information received from sensors, a tiny computer tells a miniature motor how far to move the lens in or out. Where autofocus systems diverge is in the method they use to assess the visual field—active or passive.
With an active system, the camera emits a signal in order to detect the distance of the subject from the camera, not unlike a bat using sonar to find a delicious speck of stardust. This could in fact be a high-frequency sound wave, but most of the time it’s an infrared signal. The camera sends out pulses of infrared light, which bounce off the subject and are reflected back to the camera, which registers the time this reflected signal takes to travel and moves the lens accordingly. There are some limitations to this process, however. For instance, infrared light from an open flame or a prankster’s laser pointer can confuse a sensor; dark objects can absorb the light instead of bouncing it back; and the system is most effective with subjects within 20 feet of the camera.
Passive autofocus systems, on the other hand, make their decisions based on contrast rather than distance. Under the assumption that higher contrast means a sharper image, the lens will move back and forth until it finds the distance that creates the most contrast. In fact, if you’re not sure which kind of autofocus your camera has, this property suggests an easy test. Aim your camera at a blank wall, or a patch of blue sky free of skywritten marriage proposals. Then push the shutter button halfway down. If the camera can’t focus, it’s a passive system: because there’s no contrast, the system’s little brain won’t know when to stop whirring.