What is the purpose of vertexing back to the corneal plane?

• A. To convert spectacle plane power to corneal plane power for high powers
• B. To convert corneal plane power to spectacle plane
• C. To adjust for tear film
• D. To measure pupil diameter

Answer: (A)

The main idea is that the power of a lens depends on where it sits relative to the eye. Vertex distance—the gap between the back of the spectacle lens and the cornea—changes the effective power reaching the eye. When powers are high, this distance has a bigger impact, so you convert the prescription from the spectacle plane to the corneal plane to know what the eye actually needs at the cornea. This back-vertexing lets you determine the equivalent corneal-plane power, which is essential when fitting contact lenses or planning corrections that sit on the cornea.

For example, a -10.00 D spectacle prescription at a typical 12 mm vertex distance yields a corneal-plane power closer to -8.9 D. Without this adjustment, you’d overestimate the required correction at the cornea, leading to an incorrect lens choice.

Tear film, pupil diameter, and other factors aren’t involved in this conversion, and converting in the opposite direction (corneal plane to spectacle plane) serves a different purpose.