Which statement describes curvature of field in this context?

• A. It is unrelated to the Petzval surface
• B. It is a power error when the Petzval surface does not match the eye's far point sphere
• C. It is corrected by adjusting pantoscopic tilt alone
• D. It has no relation to ray focusing

Answer: (B)

Curvature of field describes how the plane of best focus is not flat but curved as rays at different field angles come to focus at different depths. In the eye, the Petzval surface is the curved image surface produced by the optical system; if this Petzval surface does not align with the eye’s far point sphere, rays from off-axis points won’t all come to focus at the same plane. That mismatch yields a power error that varies across the visual field, so peripheral points are not in sharp focus even when the center is. Pantoscopic tilt adjusts alignment of the lens relative to the eye but doesn’t by itself fix this field-curvature error, which stems from the fundamental mismatch between the Petzval surface and the eye’s focusing plane. Ray focusing is inherently involved here because curvature of field changes where along the optical axis different field angles come to focus.