Abstract: An aperture mechanism for controlling the aperture of an optical system includes a flexible diaphragm having an opening at its center for forming a first aperture value for the optical system and means for stretching the diaphragm to enlarge the opening and form at least one additional aperture value for the optical system. A stepper motor is used to stretch the diaphragm to form a plurality of aperture values for the optical system. Because the size of the aperture is determined by how much the diaphragm is stretched, the backlash between the stepper motor and related gearing is kept to a minimum due to the elastic properties of the diaphragm, which provides economical and accurate control of very small apertures.