Certified Paraoptometric (CPO) Practice Exam

The middle value of a lens prescription is referred to as "cylindrical." This aspect of a prescription denotes the power needed to correct astigmatism, which arises from an irregular shape of the cornea or lens. The cylindrical component specifies the degree of lens power that is required at a particular axis to achieve proper focus for astigmatic patients.

This cylindrical value is essential because it complements the spherical value, which addresses nearsightedness or farsightedness. A lens prescription may contain both spherical and cylindrical components, where the cylindrical value plays a critical role in ensuring that light is properly focused on the retina despite the warped corneal shape.

In contrast, other terms like "spherical" and "prism" refer to different qualities of lens prescriptions. The spherical value corrects basic refractive errors without accounting for astigmatism, while "prism" is used to address issues of eye alignment and binocular vision rather than refractive errors.