The Dictionary Of Photography

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

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Spherical Aberration
meeting at the focus. This ring is called lateral spherical aberration, and the diameter of the same is termed the expression of the lateral aberration. The distance along the axis of the lens between the principal focus, or the focus of the central rays, R r',
Fig. 121.
atand the focus of the marginal rays, r r, atis called longi-tudinal aberration. In this case the focus of the marginal rays, being shorter than that of the central rays, the aberration is called positive spherical aberration. The circle of least confusion or least aberration, or the smallest section it is possible to make
Fig. 122.
of the cone of rays, is situated between In negative
or divergent lenses the central rays, r' r', emerge in the direction bb, and the marginal rays, R R, in the direction dd. The central rays prolonged cut the axis atwhich is then the principal focus, and the marginal rays produced cut the axis at; therefore the
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