Eyeglass prescriptions generally follow the same format, with the information displayed in a table. Abbreviations and numbers provide information for each eye that helps determine details of your new eyeglass lenses, such as their:
- Shape.
- Strength.
- Type.
Each eyeglass lens serves as a specialized tool designed to help you focus and see clear images. Your prescription shouldn’t be viewed as a measure of eye weakness or strength – instead, think of it as a customized solution that provides the optimal way to focus your eyes. Each part of your eyeglass prescription has a different designation that pertains to a different part of your eyes or vision. Here’s what it all means.
OS vs. OD
The table on your prescription typically includes two rows, one for each eye:
- OS or LE: These abbreviations indicate your left eye. OS stands for the Latin term “oculus sinister.” Some optometrists modernize their prescriptions and abbreviate “left eye” instead.
- OD or RE: “Oculus dexter” is Latin for “right eye.” Your optometrist may use “RE” instead.
The numbers listed to the right of each abbreviation relate to that eye.
Numbers in the Eye Prescription Chart
Eye care specialists determine the strength needed to correct your eyesight using diopters, a unit that measures the power of a lens. Eyesight measuring zero diopters doesn’t require any help from glasses or contacts. The farther the diopter is from zero — whether it’s positive (+) or negative (-) — the more correction you need.
The typical measurements on prescriptions generally range between -5 and +3 diopters, though some prescriptions can be stronger, measuring in the double digits in more extreme cases.
What Do the Negative and Positive Prescription Numbers Mean?
Negative numbers
Negative numbers on your prescription mean you have nearsightedness (myopia), which is trouble seeing objects or writing from a distance. Nearsightedness may affect your ability to read street signs or see details of a person or object across the street.
The number on your prescription indicates how many diopters of power are needed to correct your eyesight. If your prescription lists -2, your lens needs two diopters of power to help you see distance clearly. Nearsightedness below -5 is considered severe (high myopia).
Positive numbers
Positive numbers mean you have farsightedness and can’t see close-up objects clearly. Farsightedness (hyperopia) may make it difficult to read a restaurant menu or see your phone clearly. The higher the number, the more correction you need. Anything more than +3 is considered high hyperopia.
SPH (sphere)
Most people’s eyes are round, with the same curvature all around, like a sphere or basketball. One simple correction works for the entire eye. The number listed in the sphere (SPH) column is the overall correction you need from your glasses.
The sphere number represents the primary power of your glasses. For individuals with standard, round-shaped eyes who require correction for either distance or close-up vision, the SPH measurement might be the only number included on the prescription.
CYL (cylinder)
Approximately 33% of people in the U.S. have eyes that aren’t sphere-shaped. They have astigmatism, where the eyes are oval-shaped, like an egg or an American football. Astigmatism is a type of refractive error — a vision problem that occurs when light doesn’t enter your retina (the back of your eye) the right way.
Astigmatism is a vision issue resulting from the shape of the front of the eye. While it is possible to have astigmatism in only one eye, it is more common for both eyes to be affected.
For people with astigmatism, one universal correction won’t entirely fix their vision. Your provider will use the number in the CYL column and the SPH measurement to account for the different curves in your eye. SPH is the correction needed for the round part of your eye, and CYL relates to the correction needed for the oblong curvature in your eye. This area of the chart will be blank if you don’t have astigmatism.
Axis
The axis column also pertains to astigmatism. It explains where astigmatism is on your cornea (front of your eye) — relaying how your oblong-shaped eye tilts on a scale of 0 to 180 degrees.
An eye with a football-like shape has varying curvatures in different directions—longer and flatter in one direction and narrower with a more pronounced curve in the other. Most individuals with astigmatism typically have two primary curvatures that are approximately 90 degrees apart.
The axis measurement describes where that pronounced curvature happens. If the sharper curve points straight up — like a football standing on-end — the axis is 90 degrees. If the football is on its side, with the sharper curve pointing left or right, the axis is 180 degrees.
Add
Some people need or want two different prescriptions in one lens. Glasses with two prescription strengths have one toward the top and one toward the bottom of the lens. These glasses allow you to see clearly in multiple settings and during various tasks without having to remove or switch your glasses. Adding the number in the “add” column to the SPH number provides the second prescription (for the bottom of the lens).
Eyeglass lenses that have two separate strengths include:
- Bifocals have a clear-cut and noticeable line between your distance prescription on top and your reading prescription on the bottom.
- Progressives have a lens that slowly transitions between your distance prescription and your near prescription.
Progressive lenses, often abbreviated as PAL (progressive additional lens), incorporate the same numbers at the top and bottom as bifocals but provide a gradual transition between them rather than an abrupt change. The speed of this progression varies depending on the company or lab manufacturing the lenses.
Prism
If you have double vision (diplopia), your optometrist might prescribe prism correction. Double vision is another type of refractory error that happens when light hits two areas of the retina instead of one. Adding prism correction to eyeglasses bends the light entering the eye. The bent light falls on the correct part of the retina, producing one image instead of two.
Prism is a triangular-shaped lens designed to bend light and alter the direction of the image. Depending on the placement of the prism’s base, the image can be shifted upward, downward, to the left, or to the right.
On a prescription with prism correction, the optometrist will make a note to direct the placement of the prism base and move the image:
- BU: Move it up.
- BD: Move it down.
- BI: Move it in.
- BO: Move it out.
Cosmetically, no one can tell that glasses have prism correction. The lens may be slightly thicker if you require more than one prism.
Alternative to Eyeglasses: LASIK Surgery
While understanding your eyeglass prescription is important, some people prefer to reduce or eliminate their dependence on glasses altogether. LASIK (Laser-Assisted In Situ Keratomileusis) surgery offers a permanent vision correction solution for many people with refractive errors like nearsightedness, farsightedness, and astigmatism. With LASIK, you can potentially free yourself from having to decipher complex prescriptions and manage eyeglasses. However, it’s important to discuss with your eye care provider whether you’re a good candidate for this procedure, as not everyone is eligible for LASIK surgery.
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Reference:
Clinic, C. (2024b, December 16). Got a New Eye Prescription? Here’s What It Means. Cleveland Clinic. https://health.clevelandclinic.org/how-to-read-your-eye-prescription
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