VORRICHTUNG ZUR ERKENNUNG DES FOKUSSIERGRADES

An automatic focusing microscope projects infrared rays onto a specimen along optical paths oblique to the optical axis of an optical system. For the focusing operation, a degree of focusing in the optical system is detected according to the deviation of the reflected rays from the optical axis. A reduction optical system with a positive refractive is placed between an objective and its image plane, so that the displacement of the image plane by visible and infrared rays is reduced. In the reduction optical system, the post principal point is located between the objective and its image plane and nearer to the image plane. The focal distance is less than 1/2 the focal distance of the objective. A focusing degree-detecting means is made up of a position sensor for sensing the position of the image formed by the reflected rays. An offset signal for correcting an amount of aberration, which is based on the focal distance of the objective, is added to the output signal of the sensor, to form a focusing degree-detecting signal.