Original Auto Focus (Aug, 1971)
The Lens That Focuses Itself
Ever shoot an out-of-focus picture? Then you’ll be interested in the newest lens from Nikon. It focuses as automatically as your eye, and just as fast. You can just point and shoot at fast-moving subjects from athletes to zebras without giving focus a thought. As long as you keep your subject within the sensing circle in the center of your viewfinder, you’ll get sharp pictures. Any drawbacks? Sure. The lens is k big (11 inches long), heavy (it weighs six pounds including the batteries that power the autofocus mechanism), slow (f/4.5), and you won’t be able to buy one until next year.â€”A. J Hand
How does it work? Like this:
Light reflected from the subject passes through the first group of lens elements and is split by a ring mirror. Some of the light passes through the lens to the film plane. The rest is reflected down to the autofocus mechanism where a condensing lens forms an aerial image. The position of this image will vary according to distance of the subject. A contrast-sensing set of four photocells inside the autofocus system moves up and down the shaft of focused light. Every time the photocells pass through the point of focus (also the point of highest contrast) they send a pulse to the logic circuit. At each up-and-down cycle of the four cells, a clock pulse is fed to the logic circuit as well. A third pulse indicating the current focus of the lens also is transmitted to the circuit. The circuit takes the three pulse signals and converts them to a time signal. The time signal corresponds to the distance between the sharpest image position and the current focus position. An analog circuit and power amplifier actuate a servo motor that shifts the movable lens elements to bring the lens into focus. All this takes place several times a secondâ€”scanning, computing, and refocusing.