The vision prescription slip received after an eye examination looks like an incomprehensible matrix of fractions, Latin abbreviations, and decimal numbers. These metrics, often measured in fractions and diopters, quantify the eye’s ability to focus light and determine the exact power needed for corrective lenses. Understanding these specific measurements is the only way to gain a clear insight into one’s visual health. Demystifying the terms allows a patient to follow the progression of their eyesight and make informed decisions about their vision correction options.
How Visual Acuity is Measured (The 20/20 Standard)
The most familiar measure of vision sharpness is visual acuity, often represented by the Snellen fraction, such as 20/20. This fraction is determined using a standardized chart where the patient stands 20 feet away from a display of letters. The numerator (20) represents the testing distance in feet, and the denominator represents the distance from which a person with clinically defined normal visual sharpness can read the same line of text. Therefore, 20/20 vision means the patient can see at 20 feet what a person with normal vision can also see at 20 feet.
A higher denominator, such as 20/40, indicates poorer vision, meaning the patient must move to 20 feet to see what someone with normal vision can see from 40 feet away. Conversely, a lower denominator, like 20/15, indicates better than average visual acuity. While this fractional measurement indicates the eye’s current sharpness, it does not provide the numerical details required to grind a corrective lens.
Decoding the Prescription Slip Abbreviations
The technical language on a prescription slip uses precise abbreviations, many of which are derived from Latin, to organize the measurements for each eye. OD stands for Oculus Dexter, which specifies the correction for the right eye. OS represents Oculus Sinister, indicating the correction for the left eye.
A less common term is OU, or Oculus Uterque, which is sometimes used when the prescription is identical for both eyes. SPH is the abbreviation for Sphere, CYL for Cylinder, and ADD for Addition. The CYL correction, if present, is always accompanied by an AXIS value, which specifies the orientation of that correction.
What the Prescription Numbers Mean (Sphere, Cylinder, Axis)
The numerical values on a prescription are measured in diopters, a unit that quantifies the optical power of a lens and its ability to converge or diverge light. The Sphere (SPH) value is the primary measurement that addresses nearsightedness, known as myopia, or farsightedness, known as hyperopia. A minus sign (-) next to the SPH number signifies nearsightedness, meaning the patient struggles to see distant objects clearly, while a plus sign (+) indicates farsightedness, where near objects are blurry. The term “sphere” indicates that the corrective power is uniform across the entire surface of the lens, addressing the overall length of the eyeball.
The Cylinder (CYL) and Axis values work in tandem to correct astigmatism, a condition where the cornea or lens is imperfectly curved, often resembling the shape of a football rather than a perfectly round sphere. The CYL number specifies the lens power needed to compensate for this uneven curvature. The Axis number, which is measured in degrees from 1 to 180, indicates the precise orientation or location of the astigmatism on the eye.
If the Cylinder column is blank, the patient does not have a measurable degree of astigmatism requiring correction. The Addition (ADD) value is a separate positive power added to the distance prescription to assist with reading and other close-up tasks. This additional magnifying power is typically needed for presbyopia, the age-related loss of near focusing ability, and is used in the lower segment of bifocal or progressive lenses.