Relative Aperture
DEFINITION:
The ratio of the diameter of the effective aperture of a lens to its focal length.
DESCRIPTION:
Relative aperture is obtained by dividing the effective aperture (D) by the focal length f.
(REMARKS)
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The above definition is adopted in Japan and many countries in Europe. In the U.S.A. and England, however, relative aperture is implied by the F-number.
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In Japan, the term "aperture ratio" is also used for the same meaning as that of relative aperture. However, in England, unlike the relative aperture, aperture ratio is defined for the light beam coming from an object at a finite distance.
