The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

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Spherical Aberration
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,
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 , fig. 123, represent a convex lens; it is obvious that the marginal rays will be refracted and meet at whereas the central rays cross the axis a a atfarther from the lens ; by the insertion of a diaphragm the marginal rays are prevented from reaching the lens, and, therefore, only the central rays are used to form the image. Certain portrait lenses and others have been placed on the market, by means of which a certain amount of spherical aberration is designedly left outstanding or