Refractive Errors: What the Numbers on Your Eyeglasses and Contact Lens Prescription Mean
Key takeaways:
Changes in the shape of the eye, cornea, or lens can lead to refractive errors, which cause vision changes.
Eyeglasses and contact lenses are the most common ways to treat refractive errors.
Prescriptions for eyeglasses and contact lenses contain instructions on how to create a lens that will correct a person’s refractive error.
Table of contents
If you wear glasses or contact lenses, then you’re one of the 150 million people in the U.S. that have a refractive error. Refractive errors are the most common type of vision issue, and they’re a result of a change in how light enters the eye.
Normally, light enters through the front of the eye and bends to hit the retina in the back of the eye. Sometimes the eye isn’t able to bend, or refract, the light properly, and this leads to blurry vision.
Refractive errors can develop when:
The eyeball is too long or too short.
The cornea (clear front part of the eye) has an abnormal shape.
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Refractive errors are treated (or corrected) with eyeglasses and contact lenses. If you wear corrective lenses, you may have wondered what all the different numbers on your prescription means. Each number gives important information on your specific refractive error and what your eyes need to be able to see clearly. But those numbers don’t have to be a mystery — here’s how to read and understand your eyeglasses or contact lenses prescription.
What are the different types of refractive error?
Before jumping into prescriptions, it’s helpful to know a little bit about the different types of refractive errors:
Astigmatism: An astigmatism develops when the cornea or lens of the eye isn’t evenly curved. Normally the eyeball is shaped like a basketball. But astigmatism causes the eye to become more football-shaped.
Myopia: Myopia, or nearsightedness, develops when the eyeball is too long or there’s a problem with the cornea or lens. When light enters the eye, these changes cause light rays to land in front of the retina. This makes objects that are far away look blurry.
Hyperopia: Hyperopia, or farsightedness, develops when the eyeball is too short or there’s a problem with the cornea or lens. When light enters the eye, these changes cause light rays to land behind the retina. This makes objects that are close-range look blurry.
Presbyopia: Presbyopia develops as you get older and the lens becomes stiffer, making it harder for it to adjust when you’re looking at things up close. This makes it harder to see things that are near you, like print and screens.
It’s possible to have several refractive errors at one time. For example, you can have astigmatism, myopia, and presbyopia. So some people have prescriptions that have to adjust for several refractive errors, not just one.
How to read your eyeglasses or contact lens prescription
Prescription lenses bend the light before it enters the eye. The bend created by the lens helps to neutralize the refractive error.
Each prescription is unique because each person’s refractive error is unique, too. Your prescription lenses bend light in just the right way to fit your eye so that you can see clearly. Each item in your prescription gives information about how the light should bend before it enters your eye. Keep in mind, if you have more than one refractive error, your prescription will also be more complex.
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OD/OS
The first thing you’ll notice about your prescription is that there’s usually two sections. Each section is divided into several columns. Your prescription may not contain all of the columns mentioned on this list. But your prescription will contain two sections called OD and OS.
OD stands for oculus dexter or your right eye.
OS stands for oculus sinister or your left eye.
Your eyes may have different refractive errors. That means the numbers in your prescription will be different for your left and right eyes. This is completely normal.
Sphere
The sphere column (abbreviated as SPH) is the power in your prescription lens.
Your prescription will be either a negative or positive number:
If you’re nearsighted, your sphere will be a negative number.
If you’re farsighted, your sphere will be a positive number.
A larger number means that you need a lens that can bend more light. In other words, the bigger the number, the more powerful your prescription will be.
Your sphere number will be followed by either the word diopter, this is the unit that measures lens power. Diopter is often abbreviated as D on prescriptions.
Cylinder
The cylinder column (abbreviated as CYL) is the power needed to correct astigmatism. Your cylinder column will be blank if you don’t have astigmatism.
This number can also be positive or negative depending on the type of doctor you see. Most ophthalmologists use plus cylinder and optometrists use minus cylinder. And again, a larger number indicates astigmatism that needs more correction. Cylinder is also measured in diopters.
Axis
The axis tells the person who makes your prescription lenses where to position the cylinder power. It’s measured in degrees and will be a number between 1 and 180. So the number simply gives information about the location of the astigmatism.
Your axis column will be blank if you don’t have astigmatism.
ADD
The ADD column includes additional prescriptions. If you have presbyopia and are wearing bifocals or multifocal lens, you’ll see the additional prescriptions in the ADD column. These will be noted with a positive (+) number. Your column will be blank if you don’t have an additional prescription.
Additional columns
You’ll notice a few additional columns If your prescription is for contact lenses, including:
Brand: This is the brand of your contact lenses.
Diameter: This describes the width of your contact lenses. It’s specific to the brand and your prescription. It’s typically between 13 mm and 15 mm.
Base curve: This describes the curve of your contact lenses. The curve lets your lens sit comfortably on your eye. This is typically between 8 mm and 10 mm.
Expiration date: Contact lenses do have expiration dates. You want to make sure you use disposable contact lenses before they expire.
The bottom line
Refractive errors are the most common cause of blurred vision. They’re often treated with glasses and contact lenses. A lens prescription contains several numbers that give information on how to build the lens to correct your refractive error.
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References
American Academy of Ophthalmology. (2016). Cornea.
American Academy of Ophthalmology. (2016). Lens.
National Eye Institute. (2023). Refractive errors.
National Eye Institute. (2020). Types of refractive errors.
Porter, D. (2023). How to read an eyeglasses prescription.
