Petzval field curvature

Summary

Petzval field curvature is the aberration by which a lens images a flat object onto a curved surface instead of a plane. In a system without astigmatism this surface is the Petzval surface. Its curvature depends only on the powers and refractive indices of the elements, through the Petzval sum. It is one of the Seidel aberrations, and a flat image field requires the Petzval sum to vanish.