What would be the predicted contact lens power for a patient with Refraction: -2.00 -1.00 x 180, Ks: 43.50/44.50@090, BCR: 43.00?

• A. -1.25 D
• B. -1.50 D
• C. -2.00 D
• D. -0.75 D

Answer: (B)

Understanding how GP lens power is chosen is key: the lens you select is not simply the spectacle prescription. The cornea provides most of the eye’s focusing power (about 43–44 D on average), and a tear lens forms between the back surface of the GP and the cornea. That tear lens alters the effective power delivered to the eye, so the initial GP power is typically less negative than the full spectacle myopia, and it’s often guided by the corneal power (keratometry) and the back surface radius (BCR).

Here, the refraction shows myopic astigmatism (-2.00 -1.00 x 180). The cornea is about 43.5–44.5 D across principal meridians, with a mean near 44.0 D, while the BCR is 43.00. This pattern means the GP lens should have a spherical power somewhere between the spectacle sphere and the corneal power to account for the tear lens effect and the back surface curvature. Balancing these factors yields an initial spherical GP power of about -1.50 D (less negative than the full -2.00 D spectacle sphere), which best matches the given keratometric data and refraction.