<P> In optics, a diaphragm is a thin opaque structure with an opening (aperture) at its center . The role of the diaphragm is to stop the passage of light, except for the light passing through the aperture . Thus it is also called a stop (an aperture stop, if it limits the brightness of light reaching the focal plane, or a field stop or flare stop for other uses of diaphragms in lenses). The diaphragm is placed in the light path of a lens or objective, and the size of the aperture regulates the amount of light that passes through the lens . The centre of the diaphragm's aperture coincides with the optical axis of the lens system . </P> <P> Most modern cameras use a type of adjustable diaphragm known as an iris diaphragm, and often referred to simply as an iris . </P> <P> See the articles on aperture and f - number for the photographic effect and system of quantification of varying the opening in the diaphragm . </P> <P> A natural optical system that has a diaphragm and an aperture is the human eye . The iris is the diaphragm, and the opening in the iris of the eye (the pupil) is the aperture . An analogous dev in a photographic lens is called an iris diaphragm . </P>

The size of the opening of the diaphragm is regulated by what