An eye prescription can seem like a jumble of letters and numbers, but understanding it helps you understand your vision needs. It guides the creation of corrective lenses, tailored to your specific vision. This knowledge empowers you to better understand your eye health.
Understanding the Foundation of an Eye Prescription
An eye prescription typically begins with abbreviations that specify which eye the measurements refer to. “OD” stands for oculus dexter (right eye), and “OS” for oculus sinister (left eye). “OU” (oculus uterque) indicates the prescription applies to both eyes, often when correction is identical.
“20/20 vision” represents normal visual acuity, meaning clarity and sharpness measured at 20 feet. It indicates a person can see clearly at that distance what should normally be seen. However, 20/20 vision only reflects distance sharpness and does not include peripheral awareness, depth perception, or color vision.
Decoding Your Prescription Numbers
The core of an eye prescription lies in several key numerical components, each addressing a specific aspect of vision correction.
The “SPH” or “Sphere” value indicates the main lens power required to correct nearsightedness or farsightedness. A negative sign (-) before the number signifies nearsightedness, while a positive sign (+) or no sign indicates farsightedness. The magnitude of this number, measured in diopters, reflects the strength of the correction, with higher values indicating a stronger prescription.
The “CYL” or “Cylinder” value addresses astigmatism, a common condition where the eye’s cornea or lens has an irregular, more oval shape rather than a perfectly round one. This number indicates the amount of lens power needed to correct this irregularity, measured in diopters. If this column is blank or marked “PL” (Plano), it means there is no significant astigmatism present or it is too minor to require correction.
The “AXIS” number works in conjunction with the CYL value and is only present if there is astigmatism. It specifies the orientation or angle, measured in degrees from 1 to 180, at which the cylindrical correction should be applied to the lens. This ensures the corrective lens is precisely aligned to counteract the astigmatism’s specific angle in the eye. A higher axis number does not mean a stronger prescription; it simply denotes the position of the astigmatism.
“ADD” or “Add Power” is the additional magnifying power needed for multifocal lenses, typically for presbyopia, a natural age-related condition that affects near vision. This value, always positive, is added to the distance prescription to provide clear vision for reading and other close-up tasks. It is commonly found in prescriptions for bifocals or progressive lenses.
“PRISM” is a less common component used to correct double vision or eye alignment issues. This value indicates the prismatic power needed to shift images so both eyes can work together properly and perceive a single, clear image. Not all prescriptions will include a prism correction.
Linking Numbers to Common Vision Conditions
Negative SPH values indicate nearsightedness (myopia), where distant objects appear blurry. The higher the negative number, the greater the degree of nearsightedness. For example, a SPH of -3.00 diopters signifies a moderate level requiring significant correction.
Conversely, positive SPH values signify farsightedness (hyperopia), where close-up vision is blurry. A higher positive number indicates a more significant degree of farsightedness. A SPH of +2.00 diopters suggests corrective lenses are needed for near tasks.
Astigmatism is characterized by CYL and AXIS values. The CYL value quantifies the amount of astigmatism, reflecting the irregular shape of the eye’s cornea or lens. The AXIS value specifies the precise orientation of this curvature. For example, a prescription might show CYL -1.00 with an AXIS of 90 degrees, indicating astigmatism oriented vertically.
Beyond the Prescription: What Else Matters for Eye Health
While an eye prescription precisely details the corrective lenses needed for refractive errors, it represents only one aspect of overall eye health. A prescription primarily addresses how light focuses on the retina, correcting conditions like nearsightedness, farsightedness, and astigmatism. It does not, however, provide a complete picture of the eye’s health or detect underlying eye diseases.
A comprehensive eye exam extends far beyond determining a prescription. It involves a thorough evaluation of the entire eye, including checks for internal eye pressure, which can indicate conditions like glaucoma. Eye doctors also examine the retina, optic nerve, and other structures for signs of diseases such as cataracts, macular degeneration, or diabetic retinopathy. These evaluations are important for early detection and treatment of conditions that might not present with immediate symptoms but could lead to vision loss if left unaddressed.