Certified Paraoptometric (CPO) Practice Exam 2025

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What is the overall diameter (OAD) of a contact lens?

The distance from the center to the edge
The size of the lens when measured from edge to edge
The thickness of the lens at the center
The width of the optical zone

The correct answer is: The size of the lens when measured from edge to edge

The overall diameter (OAD) of a contact lens refers to the measurement taken from one edge of the lens to the opposite edge. This measurement is crucial for ensuring that the lens fits properly on the eye, as it affects not only comfort but also the lens's stability and how it performs in relation to the eye's movements. OAD is an important factor for optometrists and eye care professionals when determining the right size of the lens for different patients to optimize vision and comfort. In contrast, options that describe the distance from the center to the edge, thickness of the lens at the center, or the width of the optical zone do not accurately represent the overall diameter. These aspects pertain to different characteristics of the lens and are not relevant when specifically discussing the overall diameter measurement.