Active focus detecting device with infrared source

Abstract

A focus detecting device comprising a light source means capable of alternating producing a pair of infrared light bundles, a light path changing means leading the infrared lights emitted alternately from the light source means toward an objective so as to pass through the periphery of the pupil of the objective through different courses, a contracting optical system arranged between the objective or a tube lens and the image forming surface of the objective or tube lens and having a positive refractive power contracting the position deviations of the infrared light images formed in positions different with the objectives to be used, a light receiving element capable of detecting the position of the infrared light image reflected from the object and a signal processing means detecting the position deviation from the in-focus position of the objective or an imaging lens with the output signal from the light receiving element, in order to enable to obtain a high detecting precision and detecting sensitivity on the objective without requiring to adjust the lens.