If the OAD/OZD increases by 0.8 mm, by how many diopters should the base curve be flattened to maintain the same fit?

If the OAD/OZD increases by 0.8 mm, by how many diopters should the base curve be flattened to maintain the same fit?

• A. 0.25D
• B. 0.50D
• C. 1.00D
• D. 2.00D

Answer: (B)

When the OAD/OZD increases, the lens edge sits on a longer arc, which tends to loosen the fit unless the base curve is adjusted. A practical rule of thumb is that for every 0.4 mm increase in OAD/OZD, flattening the base curve by about 0.25 diopters is needed. With an increase of 0.8 mm, you apply this twice: 0.8/0.4 = 2 steps × 0.25 D = 0.50 D. Flattening the base curve means reducing its curvature (increasing the radius), so the lens maintains similar contact with the eye. Therefore, about 0.50 diopters is the appropriate adjustment.

When the OAD/OZD increases, the lens edge sits on a longer arc, which tends to loosen the fit unless the base curve is adjusted. A practical rule of thumb is that for every 0.4 mm increase in OAD/OZD, flattening the base curve by about 0.25 diopters is needed. With an increase of 0.8 mm, you apply this twice: 0.8/0.4 = 2 steps × 0.25 D = 0.50 D. Flattening the base curve means reducing its curvature (increasing the radius), so the lens maintains similar contact with the eye. Therefore, about 0.50 diopters is the appropriate adjustment.