Working Knowledge: Focusing in a Flash; August 2000; Scientific American Magazine; by Glenn Zorpette; 2 Page(s)
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.