Focus detecting device for single-lens reflex cameras

Abstract

In a single lens reflex camera a focus detecting device is provided to identify or control the in-focus condition of a lens. The detecting device operates on the same principle as that used in visual focusing, namely the bringing together of a split image or the observation of the clarity of an image or both. A group of photo-electric light detecting elements is positioned to receive a part of the object's image, which part corresponds to that watched by the photographer in determining an in-focus condition. For split image focusing the group is divided into two lines of elements located adjacent to and on opposite sides of the line splitting the image. Each line of elements effectively measures the spatial light intensity impinging thereon. When the spatial light intensity detected by both lines are the same, an in-focus condition is detected. For image clarity focusing the adjacent elements of a line of photo electric elements are connected so as to subtractively combine their photo current outputs. Thus a maximum output is achieved when the image has maximum clarity. A combination of both techniques is particularly suitable for providing increased accuracy of detection.