High plus spherical lenses usually have ______ base curves.

• A. Flat
• B. Moderate
• C. Variable
• D. Steep

Answer: (D)

High plus spherical lenses require stronger converging power, which comes from a more pronounced curvature on the lens’s front surface. That means the base curve must be steeper (smaller radius) to achieve the needed focusing effect. As the plus power increases, the front surface becomes more curved, so the base curve is typically steep to maintain the prescribed optical power while keeping the lens acceptable in thickness and optical performance. A flat base curve would not provide enough convergence for high plus powers, and a moderate or variable base curve wouldn’t reliably deliver the required power across the lens.