To benefit patients with a minus prescription, aspheric lenses can thin the edge of a high minus lens, which is achieved by?

• A. Thickening the center and steepening the back surface
• B. Thickening the edge and flattening the front surface
• C. Thin the edge of a high minus lens, steepening the front surface, flattening the back surface
• D. No change to thickness

Answer: (C)

Aspheric lens design lets us redistribute thickness and power across a lens without changing the overall prescription. For a high minus lens, the goal is to reduce edge thickness so the lens looks and feels lighter while preserving the desired minus power.

Thinning the edge is achieved by shaping the surfaces so that the edge sag is reduced. Flattening the back surface lowers its curvature toward the edge, which directly reduces edge thickness. At the same time, steepening the front surface increases the central sag, helping maintain the central optical performance and keeping the minus power from shifting too much.

If you tried to thicken the center and steepen the back surface, you would move material toward the center or edge in ways that don’t produce thinner edges. Leaving the thickness unchanged obviously wouldn’t achieve edge thinning.