Certified Paraoptometric (CPO) Practice Exam

The correct answer is A. Base curve radius. The base curve radius refers to the primary curve on the backside of the lens. This curve is essential as it determines the overall fit of the lens on the eye. A proper base curve radius ensures that the lens sits securely and comfortably on the eye, providing clear vision and optimal visual acuity.

Central thickness (B), sagittal depth (C), and vertex distance (D) are all important parameters in lens fitting and eyewear prescription, but they do not specifically refer to the primary curve on the backside of the lens.