Gas Permeable Contact Lenses – Lens Fitting, Care, And Patient Education Practice Test

When a gas-permeable lens sits on the eye, the thin tear film between the cornea and the back surface of the lens acts as its own optical layer. This tear lens usually contributes a small amount of plus power due to its curvature and thickness. So, even if you select a GP lens power based on the manifest refraction, the tear layer makes the overall optics less myopic than the lens alone would indicate. To compensate for that extra plus influence and achieve the desired retinal focus, clinicians often prescribe a slightly more minus power than the manifest refraction suggests. That tendency to push the power more negative is what we mean by over-minus GP lenses. Over-plus would hypercorrect, and equal to the cornea’s power or under-plus wouldn’t account for the tear lens’s plus contribution, so they’re not as consistent with typical GP tear-lens effects.