Learn how to read your eye prescription. Master abbreviations like OD, OS, sphere, cylinder, axis, and see why glass and contact prescriptions differ.
How to Read Your Eyeglass and Contact Lens Prescription: A Complete Medical Guide
Receiving a vision prescription from your eye care professional can feel like trying to decipher an encrypted code. A prescription is a standardized medical order detailing the exact optical measurements required to neutralize refractive errors and restore optimal visual acuity.
Understanding what these numbers, abbreviations, and symbols mean empowers you to take charge of your vision care. It also helps you understand how refractive errors alter the way light focuses inside your eye.
Deciphering the Core Abbreviations: OD, OS, and OU
The foundational structure of any optical prescription relies on standard Latin abbreviations to prevent left-to-right eye confusion:
- OD (Oculus Dexter): Refers to your right eye.
- OS (Oculus Sinister): Refers to your left eye.
- OU (Oculus Uterque): Refers to both eyes evaluated or corrected together.
Note: In modern clinical record keeping, some practitioners are shifting toward simple, universal English acronyms: RE (Right Eye) and LE (Left Eye).
Standard Components of an Eyeglass Prescription
Your eyeglass prescription uses a metric unit called diopters (D), which measures the refractive or focusing power of a lens. The farther the number moves away from zero (0.00 or Plano—meaning no correction needed), the stronger the corrective prescription required.
A standard grid contains the following clinical metrics:
1. Sphere (SPH)
This number indicates the baseline focusing power needed to correct your primary refractive error:
- A Minus Sign (-): Indicates myopia (nearsightedness), meaning your eye focuses light too far in front of the retina.
- A Plus Sign (+): Indicates hyperopia (farsightedness), meaning your eye struggles to focus light up close because it focuses behind the retina.
2. Cylinder (CYL) & Axis (The Astigmatism Correction)
If your cornea or natural crystalline lens is asymmetrical—shaped more like a football than a perfect sphere—you have astigmatism. Correcting this requires two distinct numbers that always appear together:
- Cylinder (CYL): Measures the diopter power needed to correct the astigmatism asymmetry. This value can be written in either a plus (+) format or a minus (-) format, depending on whether an optometrist or ophthalmologist wrote it.
- Axis: A number ranging from 1 to 180 degrees. It defines the exact angle where the cylindrical power must be placed in the eyeglass frame to align with the meridian of your astigmatism.
3. Reading Addition (ADD)
This column lists the additional positive magnifying power required to assist with close-up tasks like reading or sewing. It is primarily used to correct presbyopia, the natural aging process where the crystalline lens stiffens. This typically becomes noticeable between ages 40 and 45.
4. Prism and Base (Ocular Alignment)
Prism power—measured in Prism Diopters (Δ)—is prescribed when a patient experiences binocular muscle imbalances, strabismus, or fixation issues. The prism bends light paths without changing the lens focus, helping align images to eliminate diplopia (double vision). It is accompanied by a directional base setting: BU (Base Up), BD (Base Down), BI (Base In), or BO (Base Out).
Why Your Eyeglass and Contact Lens Prescriptions Are Different
You cannot use an eyeglass prescription to order contact lenses. Because eyeglasses sit roughly 12 millimeters away from the surface of your cornea, while contact lenses rest directly on your tear film, the physical power requirements change.
Furthermore, contact lens specifications require unique structural parameters to match the physical anatomy of your eye:
- Base Curve (BC): The measurement of the back surface curvature of the contact lens (typically ranging from 8.0 to 9.5 mm). It ensures the lens matches the shape of your cornea without binding or slipping.
- Diameter (DIA): The total width of the contact lens from edge to edge (typically 13.0 to 15.0 mm). This ensures the lens centers correctly over your pupil.
- Brand/Material Specifics: Contact lens orders must specify the exact proprietary material and manufacturer, as oxygen permeability and hydration levels vary significantly between brands.
Frequently Asked Questions (FAQ)
What does a "Plano" or "PL" listing mean on an eye prescription?
"Plano" or the abbreviation "PL" indicates a value of 0.00 diopters. This means you do not have a refractive error in that category, and no vision correction is required for that specific eye.
Can I convert my eyeglass prescription into a contact lens prescription myself?
No. Contact lens prescriptions require specialized spatial and structural dimensions—including Base Curve (BC) and Diameter (DIA)—along with power recalculations. These measurements can only be determined through a professional contact lens fitting by an eye care provider.
How often do eyeglass and contact lens prescriptions expire?
Most optical prescriptions expire after one to two years, depending on state regulations and individual eye health profiles. Regular expirations ensure you receive routine health checks for progressive issues like glaucoma or cataracts.
Why does my prescription have a number listed under "Add" if I am nearsighted?
If you are nearsighted and reach your 40s, your eyes naturally begin to experience presbyopia, making it harder to focus up close. The "Add" power is combined with your distance prescription to create multi-focal lenses, such as progressives or bifocals.