The numbers on an eyesight prescription serve as a standardized, universal language used by eye care professionals to quantify a vision error and prescribe corrective lenses. These measurements translate the specific way light focuses within an individual’s eye into a precise lens power. Understanding this numerical code provides insight into the nature and degree of a person’s unique refractive error. The figures determine how a lens must be shaped to refract light properly, ensuring a clear image forms directly on the retina.
Visual Acuity: The 20/20 Standard
Visual acuity refers to the sharpness or clarity of vision, typically measured using the Snellen fraction system (e.g., 20/20). The numerator indicates the distance at which the test is performed (usually 20 feet). The denominator represents the distance at which a person with normal vision would be able to read the same line of letters on the eye chart.
A visual acuity of 20/20 is the standard for normal vision. If the measurement is 20/40, the person must be 20 feet away to see what someone with 20/20 vision can see from 40 feet away. Conversely, 20/15 indicates better than average vision, meaning the person can see clearly at 20 feet what an average person can only discern at 15 feet.
Decoding Common Prescription Abbreviations
A prescription slip contains a grid of abbreviations specifying the required correction for each eye. The first set uses Latin terms: OD (oculus dexter) refers to the right eye, and OS (oculus sinister) refers to the left eye. OU (oculus uterque) is sometimes used when the prescription values are the same for both eyes.
The SPH (Sphere) column indicates the main lens power required to correct nearsightedness or farsightedness. This measurement is uniform across the entire lens surface, correcting the eye’s primary refractive error. The CYL (Cylinder) value is included if the person has astigmatism, an uneven curvature of the cornea. This value indicates the additional power needed to correct the eye’s irregular shape.
If a CYL value is present, the AXIS number must also be included, specifying the orientation of the astigmatism correction. This number is measured in degrees (1 to 180), indicating where the cylinder power is applied on the lens. ADD (Addition) represents the supplemental magnifying power needed for reading or close-up tasks. This power is typically used in multifocal or progressive lenses for people experiencing presbyopia.
PRISM is a less common entry included to correct eye alignment issues or double vision. If present, the prism value is accompanied by a directional abbreviation, such as BU (base up) or BI (base in). This guides the lens placement to redirect light and fuse the images.
Translating the Diopter Values and Correction
The actual numerical values found in the SPH and CYL columns are measured in diopters (D), which is the unit used to quantify the optical power of a lens. The sign preceding the number reveals the type of vision problem being corrected. A minus sign (-) in the SPH column signifies Myopia, or nearsightedness, meaning the person has difficulty seeing distant objects clearly.
A plus sign (+) indicates Hyperopia, or farsightedness, where the person sees better at a distance but struggles with clarity when viewing close objects. The magnitude of the diopter value indicates the severity of the refractive error; the further the number is from zero, the stronger the lens power required for correction. For example, a prescription of -5.00 D is considered stronger and represents a greater degree of nearsightedness than a prescription of -1.00 D.
Prescriptions from ±0.25 D to approximately ±2.00 D are generally considered mild, while those ranging from ±2.00 D to ±4.00 D are often classified as moderate. Values stronger than ±4.00 D indicate a more significant visual impairment, with high myopia sometimes categorized as exceeding -6.00 D. The CYL value, also measured in diopters, indicates the extent of the corneal irregularity causing astigmatism.
Astigmatism is often considered mild below 1.00 diopter, moderate between 1.00 and 2.00 diopters, and high above 2.00 diopters. This cylinder power is paired with the AXIS number, which dictates the specific angle—from 1 to 180 degrees—at which the lens must be shaped to counteract the uneven curvature of the eye. These precise numerical values ensure that the corrective lens bends light rays exactly as needed to focus a sharp image onto the retina.