Gas Permeable Contact Lenses – Lens Anatomy, Verification, and Selection Practice Test

When a GP lens sits steeper than the cornea, the thin tear film between the lens and the cornea acts like a converging (plus) lens. The posterior surface of the steeper lens is more curved than the cornea, and the tear layer that fills the gap takes on a shape that adds positive optical power to the eye. So the tear layer contributes plus power. If the lens were flatter than the cornea, the tear layer would behave as minus power; if perfectly matched, it would be near zero. This is why a steeper-than-keratometry GP lens yields a plus tear layer.