Focus detecting device for camera

A focus detecting device for a camera having a photographing lens comprising:

• means for deflecting a portion of a light beam passing through said photographing lens in a certain direction;
• a plane plate made of transparent material disposed at a predetermined angle with respect to the direction of said portion of said beam, said plane plate having first and second surfaces parallel to one another, said portion of said beam entering said plane plate at said first surface;
• a half-silvered mirror disposed upon a portion of said second surface of said plane plate at a position where said portion of said beam passing through said plane plate emerges from said second surface, a first beam passing through said half-silvered mirror and a second beam being reflected by said half-silvered mirror towards said first surface of said plane plate;
• a totally reflecting mirror disposed upon a portion of said first surface of said plane plate at a point at which emerges said second beam, said totally reflecting mirror reflecting said second beam back through said plane plate to emerge at a position away from said half-silvered mirror;
• first and second arrays of light sensors, said first array of light sensors being positioned to receive said first beam transmitted through said half-silvered mirror and said second array of sensors being positioned to receive said second beam upon emerging from said second surface.