Does increasing the OZD of a CL lens increase or decrease the sag?

• A. Increase
• B. Decrease
• C. No change
• D. It depends on material

Answer: (A)

Sag describes how deep the surface curves away from a chord. For a spherical surface with a fixed radius of curvature, the sag at a distance r from the center is approximately r^2/(2R). When the overall diameter of a contact lens is increased but the curvature stays the same, you extend farther from the center, so r increases and the sag grows quadratically. In other words, the edge of a larger-diameter lens sits deeper relative to the chord than the center does, so the sag increases.