Method and device for optical focusing

A method for optically focusing a projection image projected by a projector, comprising:

• controlling, by a detector, creation of at least one observation shot of the projection image over a time period;
• defining, by a calculating unit, at least one image region modulated over the time period with respect to at least one of (i) an optical power of the at least one image region and (ii) an optical spectrum of the at least one image region, the at least one image region being within the projection image generated by the projector;
• comparing, by the calculating unit, substantially synchronously the at least one image region of the projection image and a corresponding respective image region of an observation shot;
• calculating, by the calculating unit, a variable derived from comparisons of respective image regions and a control signal derived from the derived variable;
• and
• outputting, from the calculating unit, the control signal to a focusing device of the projector.