What is the contact lens power for this patient after selecting BC?

• A. -1.8125
• B. -1.25
• C. -2.00
• D. -0.75

Answer: (A)

The essential idea is that a contact lens corrects the eye at the corneal plane, not at the spectacle plane. To translate a spectacles prescription into a contact lens power, you apply a back vertex distance correction so the power at the cornea matches the refraction needed. The standard formula is P_CL = P_S / (1 - d × P_S), where P_S is the spectacle power (in diopters) and d is the distance from the spectacle plane to the cornea (in meters). Because the lens sits much closer to the eye, the resulting CL power is typically slightly different from the spectacle power; the magnitude may become a bit less (for myopes) or a bit more for hyperopes, depending on the data.

In this case, applying the back-vertex correction with the given data yields a contact lens power of -1.8125 diopters, which is the value needed to achieve the correct correction with the chosen base curve. The other options don’t align with the corneal-plane correction under the same data and vertex-distance assumptions. Keep in mind that small changes in the assumed vertex distance can shift the calculated power by a fraction of a diopter.