Opaque screen perforated in the center to limit the aperture of a lens or optical system by regulating the entry of the beam of light. The rays passing through the hole in the diaphragm will form the image, while the intercepted rays will not. Diaphragm types are classified by function:

Aperture diaphragm (or stop): determines the size of the opening of the optical system (linear aperture). Physically, it can be a cardboard disk with a central hole, placed on the objective of a telescope, or—for some Galilean telescopes—on the iris of the observer's eye. More often, the aperture function is performed by a lens or a component of the optical system (lens or mirror), whose edge is equivalent to the disk perforation. In photographic objectives, the aperture diaphragm consists of thin plates hinged on an external double ring. This mechanism allows the adjustment of the size of the hole (which assumes a polygonal shape). All optical systems comprise an actual aperture diaphragm or a component that acts as one.

Field diaphragm: this design is similar to the aperture diaphragm, but is lined up with an actual image inside the optical system, or sometimes placed close to the final image (thus determining the shape of the field of view). In practice, the central hole isolates the central portion of the full image, excluding the periphery (the central portion is usually brighter and better defined).

Baffle: this model is similar in shape to the previous two, but its function is to block parasitic light, for example, the light that enters obliquely through a telescope objective and is diffused by the inner walls of the tube: this produces a background of light superimposed on the image, reducing its contrast. Some telescopes are fitted with several baffles inside the tube. Another common method of reducing parasitic light—this time by absorption—is to darken the interior of the telescope. A third solution, used on some photographic lenses, is to fit the tube with an extension that acts as a shade for the objective.