Certified Paraoptometric (CPO) Practice Exam 2025

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What is the term used for the middle value in a lens prescription?

Power
Cylindrical
Axis
Base

The correct answer is: Cylindrical

The middle value in a lens prescription is commonly referred to as "power," which is a measure of how much the lens helps to focus light for the wearer. The power is typically specified in diopters and represents the strength of the lens needed to correct vision. The term "cylindrical" specifically refers to the portion of the prescription that addresses astigmatism, indicating how much additional correction is needed. "Axis" describes the orientation of the cylindrical correction in degrees. Meanwhile, "base" relates to the position of a prism if one is prescribed but does not pertain to the central value of a prescription. Thus, the correct terminology for the middle value is indeed "power."