For every 0.1 mm change in the base curve of the GP contact lens, the total power changes by approximately how many diopters?

• A. 0.25 D
• B. 0.50 D
• C. 1.00 D
• D. 1.50 D

Answer: (B)

The back surface shape of a gas-permeable lens has a strong influence on how the lens sits on the cornea and how the tear layer adds optical power. Small changes in that base curve change the effective power in roughly a linear fashion. Specifically, about 0.50 diopters of total power change occur for every 0.1 mm change in the base curve. So making the base curve 0.1 mm steeper increases the power by ~0.50 D, while making it 0.1 mm flatter decreases the power by ~0.50 D. This commonly used rule of thumb helps GP fittings predict how adjustments to the base curve will affect the prescribed correction.