field curvature

Definition: Defect or aberration of lens that causes a flat surface to be imaged onto a curved rather than flat surface. * Primary aberration in which the locus of image points of an object plane parallel to the focal plane lies on a curved -- not planar -- surface. * Also known as Petzval curvature.
Great curves, pity about the outline
Field curvature was a great problem for lens designers working in photography, made worse with the ever-present demands for wide-angle lenses. Early cameras -- notably Box Brownies -- and even the sophisticated Minox reduced the problem by bending the film with a curved film gate. Latterly, the main advance has been the increasing use of aspherical lenses. In practice, stopping down decreases the defect by increasing depth of focus.

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