Petzval curvature

Petzval curvature is an optical aberration that affects the flatness of an image field. Named after mathematician József Petzval, it causes the image of a point to form a circle, which contracts to a point at a focus displaced from the paraxial image. This displacement is proportional to the square of the image height, meaning all lines remain sharp, but the field itself is curved.1

This aberration can be partially corrected by introducing astigmatism, though this is undesirable. To achieve a flat field free of astigmatism, the Petzval sum must be significantly reduced. This is often accomplished by combining strong negative lens elements made of low-index glass with positive elements of high-index glass. The development of high-index crown glass in the 1880s was crucial for creating anastigmat lenses that addressed this issue. A related term, the Petzval lens, refers to a specific type of camera objective known for its achromatization and low transmission loss.12