Effective vergence should be considered when calculating a contact lens prescription for spectacle prescriptions exceeding how many diopters?

• A. 2.00 D
• B. 4.00 D
• C. 6.00 D
• D. 1.50 D

Answer: (B)

Vergeance at the eye changes how a spectacle correction translates to what the eye actually sees. When you move from a glasses prescription to a contact lens prescription, the lens sits directly on the cornea, so there’s effectively no vertex distance to modify the power. The power you prescribe in contact lenses must therefore be the corneal-plane (effective) power, not the spectacle-plane power.

The threshold often used is 4 diopters. Beyond this amount, the distance between the glasses and the eye (about 12 mm at a typical vertex distance) makes the spectacle power translate to a noticeably different corneal-plane power. So for prescriptions exceeding about 4.00 D, you account for effective vergence to ensure the contact lens power aligns with the eye’s actual refractive needs. For example, a spectacle +4.00 D at the corneal plane ends up around +4.20 D, illustrating why the adjustment becomes meaningful as powers rise.