Which surface is the image surface created when a system has zero radial astigmatism?

• A. Petzval Surface
• B. Fermat Surface
• C. Schwarzschild Surface
• D. Gauss Surface

Answer: (A)

When radial astigmatism is zero, the tangential and sagittal focuses align across the field, so the remaining image-plane misfocus is due to the curvature of the field itself. This curvature of the image plane is described by the Petzval surface. The Petzval surface is the curved surface on which a flat object would be in best focus; with zero radial astigmatism, the image surface corresponds to that Petzval curvature. The other surfaces aren’t the standard descriptor for this situation.