If the base curve of an RGP contact lens equals the corneal curvature, the lacrimal lens power is what?

• A. +2.00 D
• B. 0 D
• C. -2.00 D
• D. Not determinable

Answer: (B)

The tearing between cornea and a rigid gas-permeable lens acts as a lacrimal (tear) lens, whose refractive power depends on the curvature difference between the cornea and the back surface of the lens. When the back curve of the RGP matches the corneal curvature, this tear layer becomes effectively plane-parallel, contributing negligible refractive power. So the lacrimal lens power is essentially zero diopters. If there were a mismatch, the tear lens would add positive or negative power depending on whether the back curve is flatter or steeper than the cornea, but with identical curvatures the tear lens contributes no net power.