An eyeglass prescription serves as a precise map detailing the specific lens powers required to correct an individual’s refractive errors. These errors occur when the shape of the eye prevents light from focusing directly on the retina, resulting in blurred vision. The series of numbers and abbreviations printed on the prescription form a standardized system used by lens manufacturers globally to grind the corrective lenses. Understanding this code is the first step in knowing how your unique vision issues, such as nearsightedness or farsightedness, are being addressed. The values precisely indicate the degree of correction needed to allow light to properly converge and create a sharp image.
Deciphering Eye Abbreviations
Certain abbreviations designate the specific eye to which the measurements apply. “OD” stands for oculus dexter, the Latin term for the right eye. “OS” represents oculus sinister, the measurement for the left eye. Occasionally, “OU,” meaning oculus uterque (both eyes), may be used when the measurement is identical for both eyes. Establishing which eye is being referenced is necessary, as vision correction is rarely exactly the same for the right and left sides.
Understanding the Sphere (SPH) Value
The Sphere (SPH) is the foundational measurement of lens power required to correct simple nearsightedness or farsightedness. This value indicates the primary refractive error, representing the uniform degree of correction needed across the entire lens surface. Lens power is measured in units called diopters (D), which quantify the amount of light bending required to focus the image correctly onto the retina.
The SPH value always carries a sign indicating the type of refractive error. A minus sign (-) preceding the number indicates correction for myopia (nearsightedness), where a diverging lens pushes the focal point backward onto the retina. Conversely, a plus sign (+) signifies correction for hyperopia (farsightedness), where a converging lens pulls the focal point forward onto the retina. The magnitude of the number reflects the severity of the error.
Since SPH accounts for a spherical refractive error, it assumes the eye’s curvature is equal in all meridians. Myopia results from an eyeball that is slightly too long or a cornea that is too steeply curved, causing light to focus in front of the retina. Hyperopia is typically caused by an eyeball that is too short or a cornea that is too flat, causing light to focus behind the retina. The SPH value is the most common measurement and is often the only required correction for individuals without astigmatism.
Correcting Astigmatism with Cylinder (CYL) and Axis
When the eye has an unevenly curved surface, additional measurements are required to correct for astigmatism, handled by the Cylinder (CYL) and Axis values. Astigmatism occurs when the cornea or lens is shaped more like the side of a football, causing light to focus unevenly at multiple points. The CYL value quantifies the extra lens power needed to compensate for this uneven curvature.
The CYL value is measured in diopters, indicating the magnitude of the cylindrical correction required. This additional power is layered onto the spherical correction to address the difference in curvature between the eye’s steepest and flattest meridians. A zero or blank value in the CYL column means the individual requires only spherical correction.
The CYL value must be paired with the Axis measurement to specify the exact orientation of the correction. The Axis is an angle, measured in degrees from 1 to 180, that dictates where the cylindrical power must be placed on the lens. This angle corresponds to the location of the flattest meridian on the eye, ensuring the correction is applied precisely where the curvature is irregular.
The Axis value tells the lens grinder where the CYL power should be oriented, while the CYL value tells how much power is necessary. Both the CYL and Axis values must be accurate because even a small rotational error in the Axis can significantly reduce the clarity of vision.
Supplementary Power and Placement Values
Beyond the core SPH, CYL, and Axis measurements, prescriptions often include supplementary values that address specific vision needs and the physical placement of the lenses.
Add Power (ADD)
The ADD, or Add Power, is a common supplementary measurement used primarily for near vision correction. This value represents the additional magnifying power needed for reading, typically for individuals over 40 experiencing presbyopia. Presbyopia is the natural, age-related loss of the eye’s ability to focus on close objects, requiring an extra boost in power for near tasks. The ADD value is always positive and is the same for both eyes. This measurement is incorporated into multifocal lenses, such as bifocals, trifocals, or progressive lenses, to create a segment designated for reading.
Prism
A less common measurement is Prism, which is used to correct specific eye alignment issues, such as strabismus or double vision, known as diplopia. Prism diopters shift the light before it enters the eye, allowing the image from each eye to align correctly and fuse into a single image. The prescription will also specify the Prism’s base, which indicates the direction in which the lens edge is thickest (e.g., Base Up, Down, In, or Out).
Pupillary Distance (PD)
Finally, the Pupillary Distance, or PD, is a physical measurement that ensures the optical center of the lenses aligns perfectly with the center of the pupils. This distance is measured in millimeters and represents the space between the centers of the two pupils. Accurate PD is necessary for proper lens function, as misaligned optical centers can induce unintended prismatic effects, leading to eye strain or headache.