A lens is "focused" by moving its optical elements towards or away from the film until the rays of light passing through the lens produce a sharp image on the film. With SLR cameras having through-the-lens viewing, you can observe the "sharpening" of the image as the lens focus ring is turned. Non-SLR cameras accomplished the same task using super-imposed images in a viewfinder window or click-stopped zones with picture aids on the lens. Advances in computer technology in the late 1970’s enabled automating the task of focus thus making 35mm cameras, especially compacts, the cameras of choice to this day. There are two types of subject-distance detection autofocus systems available today: Infrared, often called "Active Infrared" and Phase detection used in all SLR cameras and many compact cameras, often called passive autofocus. Once the autofocus system determines subject distance, a motor is used to move the camera to the proper position for accurate focus.