a photographic copying machine wherein each of the three color filters comprises several triangular sectors which are pivotable about parallel axes into and from the path of copying light between first positions outside of the path , second positions of full overlap with the path and a finite or infinite number of intermediate positions . the pulleys for the sectors of each filter are rotatable by a discrete endless cord or cable which is further trained over a wheel receiving torque from a crank drive designed to move the sectors into the path of the light beam through progressively greater angles during movement from the second toward the first positions and through progressively smaller angles during movement from the first to the second positions under the action of a reversible stepping motor so that the crank drive can compensate for the lack of linearity between the filtering action of the sectors and the extent to which they project into the path of copying light . a device is provided to limit the extent of movement of sectors from the first toward the second positions so as to ensure that the movement of the sectors is terminated when the respective filter extends across a selected portion of the path for the light beam . a step - down gearing is interposed between the stepping motor and the crank drive .