Why is it not recommended to just flatten the base curve of a spherical plus or minus lens?

• A. It reduces edge thickness
• B. Flattening the base curve alone will cause bad optics
• C. It improves frame fit
• D. It increases weight

Answer: (B)

The base curve is part of how a lens bends light before it reaches the back surface, and it works together with the back surface and lens material to produce the prescribed vision correction. If you flatten the base curve without changing the back surface or the frame setup, the overall curvature drawn into the eye changes and the intended dioptric power is no longer delivered. This shifts the vertex power and alters how light is focused across the lens, often introducing unwanted aberrations, off-axis distortion, and prismatic effects. In short, changing only the base curve disrupts the optical balance of the lens, so it’s considered bad optics. If frame fit is the goal, adjustments should be made to frame choice or lens design rather than flattening the base curve alone, or any curve change must be accompanied by corresponding redesign of the other surfaces to preserve the Rx and image quality.