Aperture
From Encyclopædia
The aperture of a
lens, or other optical system or instrument, is the opening through which
light is admitted. The size of the aperture is limited by the size of the
lens mount. It is often desirable to be able to adjust the amount of
light passing through the
lens or device. This can be done by placing in the path of the
light rays a diaphragm, which has a hole whose size can be varied. The hole in the diaphragm then limits the size of the aperture and, hence, is called the limiting aperture or aperture stop.A more useful quantity is the effective aperture, which is the diameter of the widest pencil of rays that can enter the
lens. If the aperture stop is in front of the
lens, then the effective aperture is the same as the limiting aperture. But if the aperture stop is placed between the
lens elements, the effective aperture may differ from the limiting aperture. It is the effective aperture that determines how much
light can enter the
lens.In
photography a more widely used term is relative aperture, also known as f-
number. It is the ratio of the focal length of the
lens to the diameter of the effective aperture and is a measure of the
light-gathering power of the
lens, and thus the exposure necessary. For example, a relative aperture of f/2, f2, or 1:2 would mean that the effective aperture was half of the focal length. If a
lens of focal length 50 mm is used at a relative aperture (f-
number) of f/2, then the effective aperture is 25 mm.Increasing the f-
number, that is, going from f/4 to f/8, reduces the amount of
light entering the
camera. A wide aperture (small f-
number) lets in more
light and thus favors the use of slow film (giving
fine grain), dim
light, or brief exposures, but greatly reduces the DEPTH OF FIELD.
bibliography: Asher, Harry, Photographic Principles and Practices, 2d ed. (1975).Picture Caption[s]This design of a single-
lens reflex camera allows the photographer to see the exact image that will be recorded on the film.
light entering the
camera through the
lens is reflected into the viewfinder by a mirror, focusing screen, and pentaprism. When the shutter release is pressed, the mirror flips up and incoming
light strikes the exposed film. Other manual
Controls include the film-advance lever, film-speed dial, and rewind knob. In some modern
cameras the film is advanced and rewound automatically.
