If you have a 0.50D steeper than K lens, what is the expected tear lens?

• A. +0.50 D
• B. -0.50 D
• C. +1.00 D
• D. 0.00 D

Answer: (A)

The tear lens power depends on how the GP lens sits relative to the corneal curvature. When the lens is steeper than the cornea, the thin layer of tears between them forms a convex lens with positive power. The magnitude of that tear lens roughly matches the difference in curvature. So, if the lens is 0.50 diopters steeper than the cornea, the tear lens will be about +0.50 D. This positive tear lens adds power to the optical system. If the lens were exactly matched to the cornea, the tear lens would be near 0 D; if it were flatter by 0.50 D, the tear lens would be about -0.50 D.