A glasses prescription is a precise optical formula detailing how corrective lenses must be shaped to focus light onto your retina. While the document may look like an intimidating table filled with abbreviations and numbers, every letter and digit is a specific instruction to the lens manufacturer. Understanding this prescription is the first step toward comprehending your personal vision needs and the measurement of your refractive error.
The Essential Terminology of a Prescription
Deciphering the prescription begins with understanding the abbreviations that specify which eye the measurements refer to. The traditional Latin terms are used globally: “OD” (oculus dexter) indicates the right eye, and “OS” (oculus sinister) indicates the left eye. Occasionally, “OU” (oculus uterque) may be used if the measurement applies to both eyes, though this is less common.
Moving across the chart, “SPH” (Sphere) represents the primary lens power needed to correct nearsightedness or farsightedness. This value indicates that the correction is uniform across the entire lens surface. “CYL” (Cylinder) is present only if you have astigmatism, measuring the additional lens power required to correct the eye’s oblong curvature.
If a CYL value is present, it must be paired with an “AXIS” number, which is a degree between 1 and 180. The axis specifies the orientation or angle at which the cylindrical power must be placed in the lens to counteract the eye’s uneven curve. Finally, the “ADD” (Addition) value is the extra magnifying power needed for reading or close-up work, typically for individuals over 40 requiring multifocal lenses.
Decoding the Numerical Values
The strength of the correction for SPH, CYL, and ADD is measured in Diopters (D), the standardized unit of optical power. A diopter quantifies how much a lens must bend light to bring an image into sharp focus on the retina. Lens prescriptions are measured in increments of 0.25 Diopters, which are the smallest adjustments an eye doctor can make.
The most telling indicator is the mathematical sign preceding the Diopter value. A minus sign (-) in the SPH column signifies a need for a divergent lens, correcting Myopia (nearsightedness). Conversely, a positive sign (+) indicates a need for a convergent lens, correcting Hyperopia (farsightedness).
The magnitude of the number, regardless of the sign, directly relates to the severity of the refractive error. For example, a prescription of -4.00 D requires a much stronger lens than -1.00 D. The farther the number is from zero, the greater the degree of correction necessary.
Connecting the Prescription to Vision Conditions
The pattern of numbers and signs on the prescription directly corresponds to the four main types of refractive errors. Myopia, or nearsightedness, is diagnosed when the eye focuses light in front of the retina, often because the eyeball is slightly too long or the cornea is too curved. This condition is indicated by the presence of a negative sign (-) in the Sphere (SPH) column.
Hyperopia, or farsightedness, occurs when the eye focuses light behind the retina, typically due to a shorter eyeball or a flatter cornea. This condition is corrected with a convergent lens, noted with a positive sign (+) in the SPH column.
Astigmatism is defined by an irregularly shaped cornea, which is more oval than spherical. The presence of a measurement in the Cylinder (CYL) column, along with a corresponding Axis value, confirms astigmatism. The CYL value addresses the uneven curve, while the AXIS indicates the specific angle of that irregularity.
Presbyopia is an age-related condition where the eye’s natural lens loses flexibility, making it difficult to focus on nearby objects. This condition is exclusively indicated by a non-zero Addition (ADD) value, which provides the necessary magnifying power for close-up tasks.
Why Glasses and Contact Lens Prescriptions Differ
A common source of confusion is the difference between a glasses prescription and a contact lens prescription, as the two are not interchangeable. The primary reason for the discrepancy is a principle of optics known as vertex distance. Glasses lenses sit approximately 10 to 14 millimeters away from the eye’s surface.
Contact lenses, in contrast, sit directly on the cornea, which effectively makes the vertex distance zero. For higher prescriptions, typically those at or above plus or minus 4.00 Diopters, this physical distance change significantly alters the lens power needed for clear vision. The eye doctor must perform a calculation to compensate for this difference, which results in a lower power for nearsighted corrections and a higher power for farsighted corrections in contact lenses.
A contact lens prescription also includes unique physical measurements that ensure a proper fit on the eye, which glasses do not require. These include the Base Curve and the Diameter. The Base Curve defines the curvature of the back surface of the contact lens to match the curve of the cornea for comfort and stability. The Diameter specifies the overall size of the lens, ensuring that it covers the cornea correctly.