How does the auto-focusing system on a camera work?

How does the auto-focusing system on a camera work? — RM, Lititz, PA

There are several different systems for autofocusing. I think that the three most popular systems are optical contrast, rangefinder overlap, and acoustic distancing. The optical contrast scheme places a sophisticated light sensitive surface in the focal plane of the camera’s lens. This sensor recognizes when sharp focus is achieved by looking for the moment of maximum contrast in the image. When the lens is out of focus, the image is fuzzy and has little contrast. But when the lens is focused properly, the image is sharp and the sensor detects the strong spatial variations in darkness and brightness. The camera automatically scans the focus of its lens until it detects maximum image contrast.

The rangefinder overlap system observes the scene in front of the camera through two auxiliary lenses that are separated by a few inches. It uses mirrors to overlap the images from these two lenses and can determine the distance to the objects in the picture by the angles of the mirrors. The camera uses this distance measurement to set the focus of its main lens.

The acoustic distancing system bounces sound waves from the objects in front of the camera to determine how far away they are. The camera then adjusts its main lens for that distance. While this acoustic scheme has the advantage of working even in complete darkness, it’s confused by clear surfaces—if you take a picture through a window, it will focus on the window. The optical schemes will focus on the objects rather than the window, but they will only work when there is light coming from the objects. That’s why many autofocus cameras that use optical autofocus schemes have built in lights to illuminate the objects during the autofocusing process.

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