One or more marks (34), (36) are provided on a non-image area of a platen (23) which supports an original document (16). Electrostatic images of the document (16) and mark (34) are formed on a photoconductive drum (12) and developed to form toner images. The toner image of the original document (16) is transferred to a copy sheet (21) which is fed into contact with the drum (12) by a roller (119) or belt (19). The toner image of the mark (34) is transferred to the roller (119) or belt (19) and the optical density thereof sensed by a photosensor (39). The toner density, transfer charge and other parameters are controlled in accordance with the sensed optical density. An auxiliary sensor (91) may be provided to sense the optical density of the toner image of the mark (34) on the drum (12) prior to transfer for improved parameter control.