A vision prescription often appears as a confusing grid of numbers and abbreviations, representing the precise measurements needed to correct how the eye focuses light. Understanding this document demystifies the process of vision correction and empowers you to make informed decisions about your eyewear. This article breaks down the elements of your prescription, transforming those cryptic symbols into clear, understandable information about your visual health.
Understanding Standard Visual Acuity
Visual acuity, or the sharpness of your vision, is initially measured using a standardized eye chart, most commonly the Snellen chart. This test produces a fraction like 20/20, which is the baseline measurement for “normal” vision. The numerator, 20, represents the distance in feet at which you stand from the chart during the test.
The denominator indicates the distance at which a person with normal vision could clearly read the same line of letters. For example, a result of 20/40 means that you can only clearly read at 20 feet what a person with normal visual acuity can read from 40 feet away. Conversely, vision better than average, such as 20/15, indicates that you can see at 20 feet what an average person must move in to 15 feet to see clearly.
This visual acuity score provides a functional measure of your eyesight but does not determine the exact lens power needed for correction. The severity of vision impairment is categorized by this number, with 20/200 or worse often serving as a threshold for legal blindness, even with corrective lenses. While the 20/20 measurement is a helpful starting point, it is only one component of a comprehensive eye examination.
How Prescriptions are Determined
The final prescription numbers are the result of a two-part process that combines objective and subjective measurements of the eye’s refractive error. The objective measurement often begins with an autorefractor, a computer-controlled machine that shines a light into the eye and calculates how the light bends as it reflects off the retina. This process provides a quick, estimated starting point for the necessary lens power without requiring any input from the patient.
This initial data is then refined through a subjective test using a device called a phoropter, that distinctive instrument with multiple lenses. During this refraction assessment, the eye care professional manually switches different lens combinations in front of your eyes while you look at an eye chart. You are asked a series of questions, such as “Which is clearer, lens one or lens two?” to fine-tune the precise power needed to achieve the sharpest vision.
The full eye exam goes beyond simply determining a prescription. A comprehensive examination assesses overall eye health, checks how the eyes work together, and monitors for conditions like glaucoma. This meticulous process ensures the prescribed lenses will accurately focus light onto the retina, correcting for refractive errors like nearsightedness or farsightedness.
Decoding Your Eyeglasses Prescription
An eyeglasses prescription is organized by eye, using abbreviations derived from Latin. OD, or Oculus Dexter, refers to the right eye, while OS, or Oculus Sinister, refers to the left eye. If a value applies to both eyes, the abbreviation OU, or Oculus Uterque, may be used.
The most prominent value is the Sphere (SPH), which measures the degree of correction needed for nearsightedness or farsightedness in diopters. A minus sign (-) indicates correction for myopia (nearsightedness), meaning objects far away are blurry, while a plus sign (+) indicates hyperopia (farsightedness), where close-up objects are difficult to focus on. This spherical power is uniform across the entire lens surface.
The Cylinder (CYL) and Axis columns address astigmatism, a condition where the eye’s cornea or lens is irregularly shaped, resembling a football rather than a perfect sphere. The CYL value, also measured in diopters, quantifies the amount of power needed to correct this uneven curvature. The Axis is a number between 1 and 180 degrees that specifies the exact orientation or angle at which the cylindrical correction must be placed in the lens.
Finally, the Add (ADD) column provides the additional magnifying power required for reading or other close-up tasks, a correction commonly needed for presbyopia, which affects near vision with age. This value is always positive and is typically the same for both eyes, representing the extra power added to the lower portion of multifocal or progressive lenses. Eyeglass prescriptions typically have a power range between -20.00 and +20.00 diopters, with the ADD power often ranging from +0.50 to +3.50 diopters.
Contact Lens Specific Measurements
A contact lens prescription requires specific measurements that are not found on an eyeglasses prescription, even though the Sphere, Cylinder, and Axis values may be similar. This difference exists because a contact lens rests directly on the curved surface of the eye, whereas an eyeglass lens sits a small distance away. The proximity to the eye changes the effective power of the lens, necessitating a slight conversion in the power measurements.
Two parameters unique to contact lenses are the Base Curve (BC) and the Diameter (DIA), both measured in millimeters. The Base Curve refers to the curvature of the back surface of the lens, which must match the shape of the cornea for a proper fit. A higher BC number, typically between 8.3 and 9.0 mm for soft lenses, indicates a flatter lens, while a lower number indicates a steeper curve.
The Diameter is the measurement across the contact lens from edge to edge and ensures the lens correctly covers the cornea and sits properly on the eye. A lens with an incorrect Base Curve or Diameter can lead to discomfort, irritation, and poor visual clarity, which is why the eye care professional specifies the exact brand and material of the lens in the prescription.
The Importance of Regular Eye Exams
Interpreting your prescription is only one aspect of maintaining eye health; regular professional examinations are necessary to ensure ongoing visual wellness. For adults aged 18 to 40 who do not wear corrective lenses, an exam every two years is often recommended. This frequency increases to every one to two years once a person reaches age 40. Individuals over 65 should typically have an annual comprehensive eye exam.
These regular visits are important because prescriptions can change over time, requiring adjustments to the lens power to maintain optimal vision. The eye provides a unique view into the body’s vascular and neurological systems, allowing the eye care professional to detect underlying health issues. Conditions like high blood pressure, diabetes, and high cholesterol can show early signs in the blood vessels of the eye.
Eye exams also screen for eye diseases such as glaucoma, cataracts, and macular degeneration, which often progress without noticeable symptoms in their early stages. Early detection of these conditions is important for effective treatment and preventing permanent vision loss. Therefore, the eye exam serves as a proactive health check that extends far beyond simply determining your vision numbers.