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[Lat.,=point], the use of special signs in writing to clarify how words are used; the term also refers to the signs themselves. In every language, besides the sounds of the words that are strung together there are other features, such as tone, accent, and pauses,
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stop(diaphragm) A circular opening that sets the effective aperture (diameter) of a lens, mirror, eyepiece, etc., and also reduces stray light in an optical system.
(in optics), an opaque barrier that limits the cross section of light beams in optical systems (such as telescopes, range finders, microscopes, spectroscopes, and motion-picture and still cameras). The frames of lenses, prisms, mirrors, and other optical parts, the pupil of the eye, the edges of the illuminated object, or the slits in spectroscopes often serve as stops. The size and location of the stop determine the illumination and quality of the image, the depth of focus, and the resolution of the optical system, as well as the field of vision.
A stop that most strongly limits the light ray is called an aperture or effective stop. The image of an aperture stop in the part of the optical system in front of it determines the entrance pupil of the system; the image in the part behind it determines the exit pupil. The entrance pupil limits the expansion angle of light rays from the points of the object; the exit pupil plays the same role for rays coming from the image of the object (see Figure 1).
The illumination of the image increases with the diameter of the entrance pupil (the effective aperture of the optical system). In photographic lenses a so-called iris stop is most often used for a smooth change in the effective aperture. The ratio of the diameter of the effective aperture to the primary focal length is called the relative aperture of the lens, and it characterizes the optical efficiency of the lens or optical system. A scale that contains numbers inverse to the values of the relative aperture is usually placed on the lens frame. The use of wide light beams in fast optical systems may entail
image deterioration because of aberrations of the optical systems. Reduction to the known limit of the effective aperture of an optical system (stopping-down) improves the image quality, since fringe rays, which are most affected by aberrations, are eliminated from the light beams in the process. Stopping-down also increases the depth of focus (the depth of image space). At the same time, the reduction of the effective aperture reduces the resolution of the optical system because of light diffraction at the fringes of the stop. In connection with this, the aperture of an optical system should have an optimal value.
Other stops in an optical system primarily impede the passage through the system of rays from points of an object that lie off the principal axis of the optical system. The stop that is most effective in this regard is called a field-of-vision stop. It determines what part of space can be represented by the optical system. From the center of the entrance pupil a fieldof-vision stop is visible at the smallest angle in comparison with other stops (see Figure 2). A stop located in front of the optical system of a motion-picture or still camera is called a light blind or simply a blind.
REFERENCESLandsberg,G. S. Optika, 4th ed. Moscow, 1957. Chapter 13 §§77-79. (Obshchii kurs fiziki, vol. 3.)
Sliusarev, G. G. Geometricheskaia optika. Moscow-Leningrad, 1946.
Tudorovskii, A. I. Teoriia opticheskikh priborov, 2nd ed., vols. 1-2. Moscow-Leningrad, 1948-52.