Quantcast Aperture

Using a long-focal-length lens Focusing on near objects APERTURE The aperture, or f/stop as it is commonly called, is used to regulate the diameter of the lens opening. That controls the luminance on the film plane. Besides controlling the luminance on the film plane, the f/stop also controls image sharpness by partially correcting various  lens  aberrations. The most commonly used aperture control device is the iris diaphragm. An iris diaphragm is an adjustable device that is fitted into the barrel of the lens or shutter housing.  It  is  called  an  iris  diaphragm  because  it resembles the iris in the human eye (fig. 4-12). An iris diaphragm is a series of thin, curved, metal blades that overlap each other and is fastened to a ring on the lens barrel or shutter housing. The size of the aperture is changed by turning the aperture control ring. The blades move in unison as the control ring is moved, forming an aperture of any desired size. The control ring is marked in a series of f/stops that relate to the iris opening. The aperture controls the intensity of light that is allowed to pass to the film and the parts of the image that will appear in sharp focus. 40.163 Figure  4-12.–Iris  diaphragm. Depth  of  Field Depth of field is that zone both in front of and behind your subject that are in acceptably sharp focus. The focusing controls on most cameras are easy to use, providing you understand the factors that effect depth of field. To produce professional quality photographs, you must know how to control the depth of field. Aperture, or f/stop, is the most important factor in controlling the depth of field. The smaller the f/stop opening, the greater the depth of field; for example, at f/16, a normal lens focused on a subject 16 feet from the camera may show everything in focus from 8 feet to infinity. At f/5.6, depth of field may range from about 3 feet in front of the subject to about 6 feet behind the subject. At f/2, only the subject focused on is sharp. As shown in figure 4-3, a shallow depth of field results in a blurry foreground and background, whereas greater depth of field results in more overall sharpness. Camera-to-subject distance also has an effect on the depth of field. In general, the closer you are to the subject, the shallower the depth of field. Even at f/16 with a normal lens, if you focus on a subject only 3 feet from the camera, the depth of field may only be about 1 foot. At f/2, the subject's eyes may be in sharp focus, but the nose and ears are blurred. As you increase the camera-to-subject  distance,  the  depth  of  field  increases rapidly. Using an aperture of f/16 and focusing at 6 feet, the depth of field may extend from a foot in front of the subject to about 3 feet in back of the subject. Still using 4-10


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