Which surface curvature determines the front optic zone diameter?

• A. Radius of curvature on the anterior surface of the lens
• B. Radius of curvature on the posterior surface
• C. Base curve
• D. Optic zone diameter

Answer: (A)

The central optical region of a gas-permeable lens is defined mainly by the curvature of the front (anterior) surface. The radius of curvature of the anterior surface determines how steep or flat the central optics are and where the transition to the peripheral zones occurs, which sets the front optic zone diameter. The back surface, or base curve, is chosen to match the cornea and control fit, tear film, and alignment, but it does not fix the size of the optic zone. The posterior surface mainly influences how the lens sits on the eye, not the extent of the central optical area. So, the front surface curvature is the key factor that determines the diameter of the front optic zone.