For every 0.4 mm increase in the OZD/OAD, how should the base curve be adjusted to maintain the same fit?

For every 0.4 mm increase in the OZD/OAD, how should the base curve be adjusted to maintain the same fit?

• A. 0.10D
• B. 0.25D
• C. 0.50D
• D. 1.00D

Answer: (B)

When you change a contact lens’s diameter, you alter how the lens sits over the cornea—the sag of the lens changes. A larger diameter increases the sagittal depth, which can disrupt fit if the base curve stays the same. To preserve the same centration, lid interaction, and tear film fit, you adjust the base curve accordingly. The practical rule is to flatten the base curve by about 0.25 diopters for each 0.4 mm increase in OZD/OAD. So, for a 0.4 mm increase, you reduce the BC by 0.25D (a less curved, flatter lens) to maintain the same fit. If you were to decrease diameter, you’d steepen the BC by roughly 0.25D to compensate.

When you change a contact lens’s diameter, you alter how the lens sits over the cornea—the sag of the lens changes. A larger diameter increases the sagittal depth, which can disrupt fit if the base curve stays the same. To preserve the same centration, lid interaction, and tear film fit, you adjust the base curve accordingly.

The practical rule is to flatten the base curve by about 0.25 diopters for each 0.4 mm increase in OZD/OAD. So, for a 0.4 mm increase, you reduce the BC by 0.25D (a less curved, flatter lens) to maintain the same fit. If you were to decrease diameter, you’d steepen the BC by roughly 0.25D to compensate.