An eyeglass prescription serves as a detailed blueprint for lens manufacturers, containing numbers and abbreviations that define the precise corrective power needed for each eye. This document is the result of a comprehensive eye examination, where an optometrist or ophthalmologist determines the specific errors in how your eyes focus light. The coded information ensures that the lenses created will accurately adjust the path of light entering your eyes, allowing images to focus sharply on the retina. Deciphering these codes is the first step toward understanding your vision correction.
Identifying Left and Right Eye Abbreviations
The first step in reading a prescription involves identifying which measurements correspond to which eye. This is indicated by two Latin abbreviations standard across the optical field. OD, an abbreviation for Oculus Dexter, always refers to the right eye. The term OS, which stands for Oculus Sinister, designates the left eye.
These terms ensure that the specific vision needs of each eye are addressed accurately during lens fabrication. Occasionally, you may encounter the term OU, or Oculus Uterque, used when the same correction applies to both eyes. The distinct OD and OS notations are universally used to guide precise lens customization.
Understanding the Core Vision Correction
The primary vision correction needed is listed under the SPH, or Sphere, column, which indicates the main lens power required to correct nearsightedness or farsightedness. This power is measured in units called diopters (D), which quantify the lens’s ability to converge or diverge light. A minus sign (-) preceding the number signifies nearsightedness, or myopia, where distant objects appear blurry because light focuses in front of the retina. A plus sign (+) or no sign indicates farsightedness, or hyperopia, where light focuses theoretically behind the retina, making close-up objects difficult to see clearly. The magnitude of the number, regardless of the sign, reflects the strength of the correction; a prescription of -5.00 D is significantly stronger than a -1.00 D correction.
Astigmatism, an irregularity in the curvature of the cornea or lens, requires two additional measurements: CYL and AXIS. The CYL, or Cylinder, value represents the extra lens power needed to compensate for this uneven shape, often described as being shaped more like a football than a baseball. This value, measured in diopters, will be zero or blank if no astigmatism correction is required. The AXIS number specifies the orientation or angle (measured in degrees from 1 to 180) at which the cylindrical correction must be placed on the lens. Correctly aligning the cylindrical power along this axis ensures the light is refracted precisely to overcome the blur caused by the astigmatism.
Specialized Corrections for Near Vision and Alignment
For individuals over the age of 40, a number often appears under the ADD, or Addition, column, which addresses presbyopia, the age-related loss of near focusing ability. This number represents the extra magnifying power needed for reading or close-up tasks. The ADD value is always positive and is combined with the distance Sphere power to create the near vision correction in multifocal lenses, such as bifocals or progressives. The value for ADD is typically the same for both eyes and ranges from about +0.75 to +3.00 diopters.
A less common correction is PRISM, used to manage eye alignment problems, such as strabismus or a tendency toward double vision. Prism works by deviating light, helping the eye muscles fuse two images into a single one. The power of the prism is measured in prism diopters and is accompanied by a direction for the lens’s thickest edge, known as the base. These directions are abbreviated as:
- BU (Base Up)
- BD (Base Down)
- BI (Base In, toward the nose)
- BO (Base Out, toward the ear)
This correction precisely redirects light and alleviates strain when a person’s eyes do not naturally align on the same point.
The Essential Measurement for Lens Fitting
One of the most practical numbers on the prescription is the Pupillary Distance (PD), the physical measurement that ensures the lenses are centered correctly in the frame. PD is the distance, measured in millimeters, between the centers of the two pupils. This measurement is critical because the optical center of the lens—where the vision correction is most accurate—must align perfectly with the center of the pupil. If the PD is incorrect, even a precisely prescribed lens power can cause discomfort, eye strain, or blurred vision. The PD is sometimes given as a single number for both eyes or as two separate numbers, allowing for a more customized fit, especially in high-power or progressive lenses.