An eye prescription is a technical document containing precise instructions for creating corrective lenses. While it may look confusing, terms like SPH and DS are easily understood once decoded. This document accurately describes the specific optical power needed to correct a person’s vision errors. Understanding the abbreviations makes the prescription, which acts as a blueprint for vision correction, clear.
Decoding the Sphere Measurement
The SPH, or Sphere, measurement is the primary value on an eye prescription, indicating the main lens power required to correct vision for distance or near viewing. This measurement is expressed in units called Diopters (D), which quantify the refractive power of the lens needed to bring light into sharp focus on the retina. A higher numerical value, regardless of the sign, signifies a stronger prescription and a greater degree of vision correction is necessary.
The sign preceding the Diopter value indicates the type of refractive error being corrected. A minus sign (-) next to the SPH number denotes correction for nearsightedness (myopia), where distant objects appear blurry. Conversely, a plus sign (+) indicates the correction for farsightedness (hyperopia), meaning the person has difficulty focusing on objects up close.
The SPH value represents a lens with uniform optical power across its surface. This spherical shape corrects for issues where the eye is either too long or too short, causing light to focus incorrectly.
A value of 0.00, or “Plano” (PL), in the SPH column means no spherical correction is needed for that eye. If the prescription only contains an SPH value, the vision error is purely spherical.
Why the DS Notation Appears
The abbreviation DS stands for Diopter Sphere and is a specific notation used to communicate that the correction is purely spherical. When DS appears on a prescription, it confirms that the eye does not require a correction for astigmatism, which is an optical issue caused by an irregularly shaped cornea or lens.
The DS notation is most frequently seen in the column designated for CYL, or Cylinder, which is the measurement for astigmatism correction. When a doctor writes “DS” in the CYL column, it serves as an explicit confirmation that the cylindrical power is zero. This prevents confusion and ensures the lens is manufactured as a simple spherical lens.
Other Essential Prescription Values
While SPH addresses the eye’s overall focusing power, other measurements are included when the eye has a more complex shape. The Cylinder (CYL) value specifies the additional lens power needed to correct astigmatism. Astigmatism occurs when the eye’s surface is shaped more like a football than a perfect sphere, causing light to be focused unevenly and resulting in blurry vision at all distances.
Working in tandem with the CYL value is the Axis measurement, which indicates the precise orientation of the astigmatism correction. This is a number between 1 and 180 degrees, telling the lab exactly where to place the cylindrical power on the lens. Both CYL and Axis are required to correct astigmatism; if a CYL value is present, an Axis value must also be listed.
Another common value is ADD, which stands for Addition. This number represents the extra magnifying power needed for reading and other close-up tasks, a correction generally required due to age-related farsightedness (presbyopia). The ADD power is incorporated into multifocal lenses, such as bifocals or progressive lenses, and is typically the same positive number for both eyes.
Understanding the Prescription’s Structure
Eye prescriptions are consistently organized, typically in a table format, using abbreviations to define which eye the values correspond to. OD (oculus dexter) refers to the right eye, and OS (oculus sinister) denotes the left eye. Occasionally, OU (oculus uterque) indicates a value applies to both eyes equally. SPH, CYL, and AXIS values are listed separately for OD and OS, as each eye may require different correction.
A final, but important, measurement on the prescription is PD, or Pupillary Distance. This is the distance between the centers of the two pupils, measured in millimeters. The PD measurement is essential for ensuring that the optical center of the new lenses aligns perfectly with the wearer’s pupils for the sharpest, most comfortable vision.