A device for optically focusing a projection image projected by a projector includes a detector for controlled creation of at least one observation shot of the projection image over a time period, a processing unit for defining at least one image section within the projection image generated by the projector, where the image section is modulated over the time period with respect to the optical power and/or the optical spectrum thereof, and the at least one image section of the projection image is essentially synchronous compared with the corresponding respective image section of the observation shot, where the processing unit also calculates a variable derived from the comparison of the respective image sections and a control signal derived from the derived variable, and outputs the control signal to a focusing device of the projector.