The making of sharp picture with active autofocus
The making of sharp picture with active autofocus | ||
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Photographers can't always rely on automatic focusing because it's subject to the vagaries of any mechanism that cannot see but pretends it can. For the most part, autofocus has all but eliminated pictures of relatives with fuzzy faces and blurred birthday bashes, and it's a must for action shots and subjects who won't stand still for a portrait. The implementations of autofocus are as diverse as the minds of the ingenious engineers who invent them. We'll look here at two types of active autofocus found on less expensive cameras. One is akin to the echo technology of radar and sonar; the other is based on the triangulation used in rangefinders. When a photographer presses the shutter button, it sends a burst of electricity to a transducer on the front of the camera. A device that changes one form of energy into another, the transducer generates volleys of infrared light toward the subject of the photograph. When the infrared light bounces off the subject and returns to the transducer, it picks up each burst. The transducer turns the light energy into electrical currents that tell the camera's circuitry how long it took the infrared light to travel from the camera to the subject and return. The round-trip time for the light takes a little under 2 nanoseconds for each foot from the camera to the subject. The measurement of that time goes to a microprocessor that controls a small motor built into the lens housing. The motor rotates to a position that's been calibrated to focus on an object at the distance determined by the infrared bounce. This type of autofocus works with subjects no more than about 30 feet from the camera. With any subject farther than that, the returning light is too faint to register. In that situation, the camera sets the lens to infinity, which brings into focus any subject from 30 feet to the farthest star. | ||