If Mary has a base curve of 43.50D and a power of +2.75D on her gas permeable lens, what would be the new power required if the base curve changes to 43.00D?

To determine the new power required when changing the base curve of a gas permeable lens, it's important to understand how base curve and power are related. The relationship is influenced by the effective lens power formula, which takes into account the curvature of the lens in relation to the front surface.

When shortening the base curve from 43.50D to 43.00D, the lens is becoming steeper. A steeper base curve typically increases the effective power of the lens. The amount of change in lens power corresponding to a change in base curve can be estimated using a rule of thumb: for every 0.1 mm change in base curve, the lens power may change by approximately 0.25D.

Here, the change in base curve is 0.50D (going from 43.50D to 43.00D, which is a steeper curve). To find the change in power:

• Change in power = (Change in base curve in mm) x (0.25D per 0.1 mm)
• The change in base curve is 0.5D, so the adjustment in power would be 0.5D x 2.5 (since 0.5 mm