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Field Curvature

Field curvature characterizes the natural tendency of optical systems to have curved image planes. A positive singlet has an inward bending image surface.

A perfect image is formed on a curved surface, and the image blur at the paraxial image plane increases as H². A compromise flat image plane that reduces the average image blur occurs inside paraxial focus.

The field curvature is a bias curvature for the astigmatic image surfaces.

While not a good image surface, the Petzval surface represents the fundamental field curvature of the system. It depends only on the construction parameters of the system: surface curvatures and element indices of refraction.

These four image surfaces are equally spaced and occur in the same relative order: T-M-S-P or P-S-M-T. The best image quality occurs at medial focus. An artificially flattened field or medial surface can be obtained by balancing astigmatism and field curvature.