LensForge Icon

Petzval sum

The Petzval sum P is the negative of the sum over glass interfaces of the interface curvature times the change in reciprocal refractive index across the interface:

petzval

Because the Petzval sum is a property of the construction parameters, and not the path that the rays take through the system, it can be evaluated without ray tracing.

The Seidel aberration S4 (field curvature) is related to the Petzval sum by S4 = PH2, where H is the Lagrange invariant. If only this aberration is present, then the image is sharp on the Petzval surface, whose curvature is equal to -P.

See also