The Dictionary Of Photography

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

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Curvature of the Field
theoretically, only the point e would be in focus. A concave or negative lens has, as is well known (see also Chromatic a
Aberration), no real focus, and the marginal rays are more refracted than the central, so that by combining a concave a
with a convex lens we lengthen the focus of the marginal rays, and thus flatten the field. The use of a stop also still further increases the flatness of field, as shown in fig. 39.