If you have a 2.00D flatter than K lens, what is the expected tear lens?

• A. +2.00 D
• B. -2.00 D
• C. 0.00 D
• D. -1.00 D

Answer: (B)

The tear lens power reflects the difference in curvature between the cornea and the contact lens, with the sign opposite to which surface is steeper. If the GP lens is flatter than the cornea by 2 diopters, the tear film between them behaves like a minus lens of the same magnitude to balance the optical system. So the tear lens would be -2.00 D.

In contrast, a positive tear lens would occur if the lens were steeper than the cornea, a zero tear lens if the curvatures match, and a 1.00 D difference would yield a tear lens of -1.00 D.