Which type of Spherical aberration occurs when the Petzval surface does not correspond to the far point sphere of the eye?

• A. Longitudinal
• B. Radial
• C. Curvature of Field (Power Error)
• D. Distortion

Answer: (C)

When the Petzval surface does not line up with the eye’s far point sphere, the focus across the image field cannot lie on a flat plane. The Petzval surface describes how the focal surface bends due to lens design; if it doesn’t match the eye’s far point sphere, rays from off-axis points come to focus at different depths than those from the center. The result is an image that blurs more toward the edges because the best focus forms on a curved surface rather than the flat retina plane. This tendency is called curvature of field, also referred to as a power error in optical terms. It’s different from longitudinal spherical aberration, which is a shift of focus along the optical axis for rays at different apertures, or from distortion, which changes magnification with field position.