If you steepen the base curve of a rigid gas-permeable lens by 0.1 mm, how much power must be added to the lens?

If you steepen the base curve of a rigid gas-permeable lens by 0.1 mm, how much power must be added to the lens?

• A. +0.50 D
• B. +0.25 D
• C. -0.50 D
• D. -0.25 D

Answer: (C)

Steepening the base curve changes how the lens sits on the eye and how the light is refracted by the lens surfaces. For rigid gas-permeable lenses, a small change in the back-curve radius has a relatively large effect on the lens’s effective power. Specifically, increasing the steepness by 0.1 mm reduces the lens’s converging power by about 0.5 diopters. To maintain the same overall optical correction, you would need to add negative power to the lens—about -0.50 D. So a 0.1 mm steepening corresponds to roughly a -0.50 D adjustment.

Steepening the base curve changes how the lens sits on the eye and how the light is refracted by the lens surfaces. For rigid gas-permeable lenses, a small change in the back-curve radius has a relatively large effect on the lens’s effective power. Specifically, increasing the steepness by 0.1 mm reduces the lens’s converging power by about 0.5 diopters. To maintain the same overall optical correction, you would need to add negative power to the lens—about -0.50 D. So a 0.1 mm steepening corresponds to roughly a -0.50 D adjustment.