Spherical Aberration |
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focus of the marginal rays is longer than that of the central rays, and this is termed negative spherical aberration. The sphericity of the surfaces of a lens being the cause of spherical aberration, it necessarily follows that the more curved these surfaces are relative to the diameter of the lenses the greater is the spherical aberration ; thus, lenses of comparatively short focus and large aperture, like portrait lenses, are more likely to suffer from spherical aberration. Any divergent or convergent lens suffering from this defect, in a greater or less degree, may be cured to a great extent by placing a diaphragm or stop in front of the lens, |
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Fig. 123.
which actually reduces the lens to a smaller working aperture, and thus prevents the marginal rays from being refracted by the lens. The use of the diaphragm, however, will not completely cure spherical aberration. It reduces it, it is true, to a negligable quantity; thus let
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