mirror lens

optics
Also known as: catadioptric lens

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design and use

  • sequence of negative–positive process
    In technology of photography: Mirror lenses

    Images can also be formed by light reflected from curved mirrors. This method, long used in astronomical telescopes, is applied to long-focus lens systems of short overall length by folding the light path back onto itself. A mirror lens or catadioptric system has…

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lighthouses

  • lighthouse
    In lighthouse: Rectangular and drum lenses

    Thus emerged the full Fresnel catadioptric system, the basis of all lighthouse lens systems today. To meet the requirement for a fixed all-around light, in 1836 English glassmaker William Cookson modified Fresnel’s principle by producing a cylindrical drum lens, which concentrated the light into an all-around fan beam. Although not…

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telescope

  • Keck Observatory
    In telescope: The Schmidt telescope

    A catadioptric telescope design incorporates the best features of both the refractor and the reflector—i.e., it has both reflective and refractive optics. The Schmidt telescope has a spherically shaped primary mirror. Since parallel light rays that are reflected by the centre of a spherical mirror are…

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Related Topics:
lens
camera
film

speed, in photography, any of those standards that indicate (1) the size of the lens opening, or aperture, (2) the duration of exposure, and (3) the sensitivity of the film to light.

The aperture, or lens speed, of a camera is the size of the opening in the lens. Aperture settings provide one means of controlling the amount of light that falls on the film by determining the maximum diameter of the light beam entering the camera body. A standard set of numbers, called f-stops, describe the lens aperture as a ratio of the focal length (see relative aperture).

The shutter speed regulates the length of time that the shutter is open during an exposure. Varying the shutter speed controls the film’s exposure to light and determines the speed of action that the photograph can “freeze,” or reproduce without blurring the image. Shutter speeds generally range from one second to 1/2,000 of a second.

Film speed indicates the sensitivity of a particular emulsion to light. It is usually expressed as an ISO (International Standards Organization) number (formerly called, and identical to, the ASA [American Standards Association; now American National Standards Institute] number), or, in Europe, as a DIN (Deutsche Industrie Norm) number. A film with a speed of 32 ISO/ASA, or 16/10 DIN, for example, would be considered a slow film—i.e., relatively insensitive to light, and most effectively used in bright light—whereas one with a speed of 400 ISO/ASA, or 27/10 DIN, would be considered fast—i.e., relatively sensitive to light and therefore usable in dim light or with a very fast shutter speed.

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