A photographic objective, used for reproduction purposes in a darkroom, has an iris diaphragm whose control ring is mechanically coupled with a movable arm sweeping a contact arc inside the camera; the setting of the diaphragm can be read on an external indicator, connected to the contact arc and the arm through a cable, which includes a light box emitting filtered light to which the copying paper is insensitive. The contact arc, which may be a set of bank contacts or a continuous resistance, is carried on an insulating plate which may be removably seated in a slot of the camera housing with a plug-in connection.