Diaphragm in Optical Instruments

DIAPHRAGM IN OPTICAL INSTRUMENTS is an opaque screen with an aperture that limits the boundaries of a light beam in an optical system. The purpose of a diaphragm can be various. By reducing the size of the diaphragm, distortions of the optical image caused by spherical aberration are eliminated to a significant degree. The depth of focus and the depth of field of the image depend on the size of the diaphragm. Both increase as the diaphragm, appropriately positioned in the optical system, is reduced. Finally, the diaphragm regulates the light-gathering power of the system and can be used in photometric setups to attenuate light by a specific factor.

The simplest type of diaphragm is the lens mount, but in addition, special diaphragms, almost always with a circular aperture, are used in microscopes, telescopes, photographic cameras, and other optical instruments. Besides fixed diaphragms, diaphragms with variable apertures are often used. The most perfect among such variable diaphragms is the iris diaphragm. It consists of a system (12–16) of very thin metal or ebonite plates of sickle shape, arranged in different planes but very close to one another. By moving a common handle, the plates can be shifted in such a way that their edges leave open an aperture of almost circular shape and variable area (see figure).