A contact lens prescription is a specialized medical document detailing the exact specifications required for a corrective lens to rest safely and effectively on the surface of your eye. Unlike a glasses prescription, which corrects vision from a distance, the contact lens prescription includes additional measurements to ensure the lens correctly fits the curve and size of your cornea. Understanding the abbreviations and numbers on this document empowers you to purchase the correct lenses and ensures the ongoing health of your eyes.
Identifying Which Eye is Which
The first step in decoding the document involves identifying which set of measurements applies to each eye using traditional Latin abbreviations. “OD” (Oculus Dexter) designates the right eye, and “OS” (Oculus Sinister) designates the left eye. Occasionally, you may see “OU” (Oculus Uterque), meaning both eyes, though this is less common for individual measurements. The prescription details for the right and left eyes are listed separately because vision correction or physical dimensions often differ between them.
Standard Lens Power and Fit Details
The core of any prescription is the lens power, indicated by SPH or PWR (Sphere or Power). This value, measured in diopters, represents the corrective strength needed for nearsightedness or farsightedness. A minus sign (-) signifies correction for nearsightedness (myopia), while a plus sign (+) indicates correction for farsightedness (hyperopia). The greater the absolute value of this number, the stronger the required correction.
Unlike glasses prescriptions, contact lens prescriptions include physical fit parameters. The Base Curve (BC) dictates the curvature of the back surface of the lens, which must closely match the curvature of your cornea for a safe and comfortable fit. This measurement is typically a number between 8 and 10 millimeters. The Diameter (DIA) indicates the overall size of the lens from edge to edge, measured in millimeters. The diameter ensures the lens properly covers the cornea and rests correctly on the eye.
Correcting for Astigmatism
If you have astigmatism (an irregularly shaped cornea), your prescription will include two additional parameters for specialized toric lenses. CYL (Cylinder) measures the extra power needed to correct the astigmatism, indicating the severity of the corneal irregularity in diopters. AXIS specifies the orientation or angle at which the cylindrical correction must be applied to the eye. This value, represented by a number between 1 and 180 degrees, ensures the lens is oriented correctly. If the CYL and AXIS fields are blank or read “DS” (Diopters Sphere), you only require a standard spherical lens correction.
Understanding Multifocal and Specialty Lenses
For those requiring correction for both distance and near vision, often due to presbyopia, the prescription includes an ADD power (Addition or Added Power). This is the magnifying power layered onto the primary distance power to help with close-up tasks like reading. The ADD power is usually a positive number, though some brands may list this as “Low,” “Medium,” or “High” instead of a specific diopter value.
For multifocal or monovision lenses, the prescription may specify the Dominant (D) and Non-Dominant (N) eye. This designation is important because the lens design prioritizes distance vision in one eye and near vision in the other to achieve a blended visual outcome. A contact lens prescription is always brand-specific, listing the exact manufacturer and product name, as fit and material properties vary significantly. Finally, the prescription must include an expiration date, which legally requires a new eye examination and contact lens fitting to renew authorization to purchase lenses.