How is the power of a patient's contact lenses determined?

• A. Power of the lenses vertexed back to the corneal plane
• B. Power of the lenses measured at the spectacle plane
• C. Power of the lenses adjusted by astigmatic correction
• D. Power of the lenses determined by age

Answer: (A)

Translating the prescription from the spectacle plane back to the corneal plane by vertexing is how contact lens power is determined. Glasses sit about 12–14 millimeters away from the eye, so the optical power needed at the cornea is slightly different from the power printed for glasses. By applying the vertex distance, the spectacle prescription is adjusted to reflect what the lens on the eye must do to achieve the same refractive effect. This corneal-plane power is what guides the actual contact lens power.

Astigmatic correction and axis come into play for reducing blur from multiple meridians, but they don’t set the base power itself. Measuring the power at the spectacle plane or factoring in age isn’t how the lens power is determined for contact lenses. The key step is converting the prescription to the corneal plane through vertexing so the lens provides the intended true refraction on the eye.