Photography Autofocus Systems

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Focusing in a Flash

High-quality cameras all employ the same basic system for automatic focusing, known as phase-detection autofocus. In a single-lens reflex (SLR) camera, light from a part of the scene passes through the lens and then through the camera's reflex mirror, which is partially transparent. (The reflex mirror flips up when the shutter button is pressed to allow the image to fall on the film.) A submirror, attached to the back of the reflex mirror, directs the light that comes through the main mirror downward to the autofocus module.

After passing through various lenses and filters, the light rays fall on an array of light-sensitive charge-coupled devices (CCDs). It is the distance between the illuminated CCD elements that indicates how close the image is to being in focus. A logic circuit constantly monitors that separation and drives a motor that spins the focusing ring of the lens, shifting the focus. When the separation hits a predetermined value, the logic circuit stops the motor and flashes lights in the viewfinder to indicate that the image is focused.

The basic scheme for phase-detection autofocus was the subject of patents awarded in the U.S. to Honeywell in the 1970s. Honeywell's technology was first used in the Konica C35 AF, a point-and-shoot camera that sold briskly but briefly in the late 1970s. In comparison with today's systems, this early implementation was slow and easily stymied by low light or low contrast. In the mid-1980s Japanese camera manufacturers, which had been working on autofocus systems for about 15 years, began introducing improved phase-detection autofocus systems in their models sold in the U.S.

Honeywell, in a landmark patent-infringement case noted for its nationalistic overtones at a time when fears of Japanese technological dominance were cresting in the U.S., sued Minolta over the Maxxum 7000 camera. Ultimately, in 1992, after Minolta lost the jury decision, the company had little choice but to pay Honeywell $127.5 million to license the autofocus system and a related low-light flash technology. Other Japanese camera makers were then forced to make similar deals with Honeywell.

The relatively poor performance of early autofocus systems earned the technology a bad name among professional photographers, who resisted it for years. Great improvements in speed and reliability finally won over even that market segment, and today autofocus is a standard feature on 35-millimeter cameras ranging from modest point-and-shoot units to top-of-the-line professional models.

—Glenn Zorpette, staff writer

HIGH-END SLR cameras tie the autofocus to other functions. Most can take a light-meter reading of a "spot" area, just a few percent of the image. In these models, the autofocus can be coupled to the light meter, so that the unit focuses on the subject while simultaneously taking a spot-meter reading of it. Sophisticated software permits the system to pickout the probable subject. Flashing lights in the view screen indicate which of the several autofocus arrays is activated, revealing which object the camera is focusing on.

PENTAPRISM

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