The series of stops usually employed is
calculated so that each aperture is half the area of the preceding.
Stated in terms of the focal length they are known as F/5.6, F/8,
F/11.3, F/16, F/22.6, F/32, &c. Since the squares of those numbers,
31.4, 64, 127.7, 256, 510.7, 1024 are approximately each twice the
preceding number of the series, the apertures, F(ocal length),
divided by the successive numbers as denominators, are each half the
area of the preceding and require twice the exposure, F/16 requires
twice the 'exposure of F/11.3, and four times that of F/8, and so
throughout the scale.
Stops are used to regulate either 'depth of focus' or length of
exposure. The 'depth of focus' means the distance before and behind
the point in theoretically accurate focus, at which objects are
sufficiently focussed, for the purpose the photographer has in view.
This length is greatest when only the central portion of the lens is
in use. It is greatest with a pinhole, and least with a full
aperture. Hence a small stop is required if the picture is to include
near and far objects, while a large aperture may be used if all the
subject is far enough away to be in clear focus--say more than 25
feet--or if it is a flat surface. The small stop is also required when
the rising front or the swing back is in use.
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