An eye prescription often appears as a dense grid of abbreviations and numbers that can be confusing. This document is a precise formula for correcting refractive errors that prevent light from focusing correctly on the retina. Astigmatism occurs when the cornea or lens has an irregular, football-like curvature instead of a perfectly spherical shape. This irregularity causes light to scatter, leading to blurred or distorted vision. Understanding how this error is noted on your prescription is the first step toward decoding your vision health.
The Standard Prescription Format
The structure of an eye prescription is standardized to ensure accuracy when ordering corrective lenses. Information is organized first by the specific eye being measured: “OD” (Oculus Dexter) for the right eye, and “OS” (Oculus Sinister) for the left eye.
Following the eye designation, the prescription uses columns representing the distinct measurements required to manufacture the lens. The most common column headings are Sphere (SPH), Cylinder (CYL), and Axis (AX). These three primary fields define the optical power needed for each eye and indicate the presence and severity of various refractive errors.
The Astigmatism Indicators: Cylinder and Axis
The key to identifying astigmatism lies in the Cylinder (CYL) column. This number represents the extra lens power required to correct the irregular curvature of the eye, measured in diopters (D). If the CYL field contains a value other than zero or “Plano,” astigmatism is present. The magnitude of this number indicates the severity.
The Cylinder value is always accompanied by a number in the Axis (AX) column. The Axis measurement specifies the precise orientation or angle of the astigmatic curvature, expressed in degrees ranging from 1 to 180.
Both the Cylinder power and the Axis angle are required together because astigmatism correction must be applied to a specific orientation. The sign preceding the Cylinder value (positive or negative) relates to the prescribing style.
Distinguishing Astigmatism from General Vision Correction (Sphere)
The Sphere (SPH) column addresses the eye’s overall ability to focus light, correcting for nearsightedness (myopia) or farsightedness (hyperopia). A negative (-) number indicates myopia, while a positive (+) number signifies hyperopia.
The Sphere power is uniform across the entire lens, providing a single power change to shift the focal point. This differs from the Cylinder measurement, which provides power only along a specific axis to correct shape irregularity. If the Cylinder field is populated, the prescription includes both general focus correction and astigmatism correction.