Astigmatism is a common condition that occurs when the eye’s front surface, the cornea, or the lens inside the eye has an imperfect curvature, shaped more like a football than a basketball. This irregularity causes light to focus on multiple points instead of a single point on the retina, resulting in blurred or distorted vision. The prescription form clearly isolates the exact measurements needed to correct astigmatism. These numbers are separate from those that correct for simple nearsightedness or farsightedness.
Deciphering the Prescription Layout
The typical eyeglass prescription is set up as a grid, providing specific correction values for each eye. The rows are labeled with Latin abbreviations: OD (Oculus Dexter) for the right eye, and OS (Oculus Sinister) for the left eye. Sometimes OU (Oculus Uterque) indicates a measurement that applies to both eyes. The first major column is usually marked Sphere (SPH), which indicates the lens power needed to correct basic nearsightedness or farsightedness. This SPH value is measured in diopters and will have a plus sign (+) for farsightedness or a minus sign (-) for nearsightedness.
The Cylinder and Axis Values
The numbers that specifically address astigmatism fall under two columns: Cylinder (CYL) and Axis (AXIS). The CYL value quantifies the lens power required to neutralize the irregular curvature of the eye. This measurement is expressed in diopters and always carries a plus (+) or minus (-) sign. Because astigmatism is directional, the AXIS number specifies the orientation of the astigmatism on the eye as an angle (1 to 180 degrees), telling the lens manufacturer where to position the correction. If the CYL column is blank, or sometimes marked “SPH,” it means you do not have a measurable astigmatism that requires correction.
Gauging the Degree of Astigmatism
The magnitude of astigmatism is determined solely by the number found in the Cylinder (CYL) column. This diopter value indicates the strength of the corrective lens needed to compensate for the eye’s shape; a higher numerical value signifies a greater degree of astigmatism. Severity levels are based on CYL power: mild is less than 1.00 diopter, moderate astigmatism is 1.00 to 2.00 diopters, and severe is 2.00 diopters or higher. The Axis number is essential for grinding the lens but does not relate to the degree of the condition. It dictates the precise angle at which the cylindrical power must be placed.