most modern cameras measure the subject luminance and set the shutter or the lens diaphragm to yield a correctly exposed image. Professional photographers also use hand-held separate exposure meters and transfer the readings manually to the camera. In terms of the seemingly innocent brown bag that has been used to pack lunches for decades, the term actually traces back to the brown paper bag test, which was traditionally used to judge skin color by certain African-American sororities and fraternities. The upper section of a twin-lens reflex camera is such a reflecting finder. The focusing screen is often overlaid by a pattern of fine concentric lens sections. The most elaborate camera systems also include such accessories as alternative finder systems; interchangeable reflex screens, film backs, and magazines; and remote-control and motor-drive systems. New York: The Metropolitan Museum of Art, 2007. This early "photogenic drawing" process was a printing-out process,.e., the paper had to be exposed in the camera until the image was fully visible. London: Hering Remington; Peterborough,. Some aberrations can also be corrected by making one or more of the surfaces of a lens system aspheric ;.e., with the variable curvature of a paraboloid or other surface rather than the constant curvature of a spherical one.
On turning the mirror through the correct angle, which depends on the object distance, the two images are made to coincide. In late 1840, Talbot worked out a very different developing-out process (a concept pioneered by the daguerreotype process introduced in 1839 in which only an extremely faint or completely invisible latent image had to be produced in the camera, which could be done. Zoom lenses for still cameras have focal-length ratios from 2:1 to 4:1 or more (.g., 35135 mm for a 35-mm reflex). It can be made to open for a predetermined time to expose the film to the image formed by the lens. Selenium cells, still used in some cameras, also generate a current but are larger and less sensitive. The reference list research paper apa resulting contrast-transfer curve, or function, gives a more reliable indication of the lens performance under practical picture-taking conditions. Such defects, called aberrations, can be eliminatedand even then not completelyonly by replacing the single lens element by a group of elements of appropriate shape and separation. The most common flash system depends on a high-voltage discharge through a gas-filled tube. The squares of the f -numbers are inversely proportional to the amount of light admitted. Special lens types Apart from general-purpose camera lenses of various focal lengths, there are lenses of special characteristics or design. A low-voltage circuit generating a high-voltage pulse triggers the flash, which lasts typically 1/1,000 second or less.
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