Certified Paraoptometric (CPO) Practice Exam

The axis of the lens is an essential term in optical prescriptions, particularly concerning cylindrical lenses. The cylinder's axis refers to the orientation of the cylindrical correction in the lens, which is necessary for correcting astigmatism. The axis is measured in degrees and indicates where the greatest curvature of the lens is positioned, allowing the light rays to bend accordingly to improve vision.

In contrast, the other options, such as the surface, center, and edge, refer to different physical aspects of the lens rather than its corrective function for astigmatism. The surface of the lens pertains to the shape and texture of the lens, while the center typically denotes the middle point of the lens where it is thinnest. The edge indicates the outer perimeter of the lens, which does not provide information regarding the cylindrical correction's orientation. Thus, understanding the axis is fundamental for proper fitting and effectiveness of corrective lenses in astigmatic patients.