What does the "CYL" value represent in a contact lens prescription?

The "CYL" value in a contact lens prescription specifically denotes the amount of astigmatism correction needed for the patient. Astigmatism is a common refractive error caused by an irregular curvature of the cornea or lens, which results in blurred or distorted vision. The CYL measurement indicates the degree of this irregularity, expressing how much additional power is required in a specific meridian of the lens to provide proper focus.

In practical terms, the CYL value is essential for creating a lens that adequately compensates for the uneven curvature, allowing patients with astigmatism to achieve clearer vision. The accompanying axis value (often provided in degrees) specifies the orientation of this correction. This is crucial information that differentiates astigmatism correction from other types of refractive corrections, such as those for nearsightedness or farsightedness, which are indicated by the spherical (SPH) component of the prescription.