Abstract: An automatic focusing device for slide projectors in which at least one photo-electric receiver of the secondary projection device which determines the position of the slide generates, from a beam of light impinging upon the receiver, the error voltage controlling the focusing of the slide. The error voltage is fed into an electronic amplifier circuit of the focusing device, which contains manual facilities for refocusing the slide comprising primary switching elements which automatically apply the error voltage produced by the photoelectric receiver to the positioning motor for the focusing device, but only when this error voltage reaches a predetermined level.