In an aspherical lens, which surface(s) flatten toward the edge?

• A. Front surface only
• B. Back surface only
• C. Front and back curves (both)
• D. Neither surface

Answer: (C)

Aspheric surfaces change shape from the center of the lens to the edge, rather than being perfect spheres. Flattening toward the edge means the curvature becomes gentler as you move outward, so marginal rays travel with less deviation than they would on a strictly spherical surface.

When both sides—the front and the back—are designed with this edge-flattening tendency, the lens minimizes spherical aberration across the full optical zone. The center can provide the needed power with a steeper curvature, while the edges become flatter to keep peripheral rays focused more closely to the paraxial focus. This combined adjustment on both surfaces yields better image quality from center to edge and reduces distortion and blur in the periphery.

If only one surface were aspheric with edge flattening, the improvement would be present but less comprehensive; having both surfaces flatten toward the edge maximizes the corrective effect across the entire lens.