The ____________ Surface is the image surface created by a system with no radial astigmatism.

• A. Petzval Surface
• B. Gaussian Surface
• C. Fresnel Surface
• D. Principal Surface

Answer: (A)

The image surface for a system with no radial astigmatism is the Petzval surface. Field curvature means that different field heights come to focus at different depths, so a flat image plane isn’t the best focus for all points. If radial astigmatism is eliminated, the tangential and sagittal focuses line up across the field, and the best focus lies along a curved surface—the Petzval surface. This curvature is inherent to the lens arrangement and defines the natural image plane produced by the system. In practice, correcting field curvature to a flat plane requires additional elements, but the curved locus itself is the Petzval surface.