You’ve gone to the optometrist, had your eye exam, and now it’s time to get your prescription. When you look at it, though, you’re perplexed—what do all these letters and numbers mean? How do they tell you what kind of glasses you should get?
Not to worry. Eye prescriptions can be confusing, but we’ll explain all of their different parts so you know how to read one. Not only will it help you understand your eyewear better, it also makes for an extremely practical party trick.
Your Glasses Prescription, Explained
Whether it’s on paper or digital, your eye prescription probably looks like a small grid or chart with rows, columns, and cells. Confusingly, the chart incorporates a mix of letter abbreviations, positive and/or negative numbers, and full words.
What do they all mean? Consult our glossary below to tackle each element, then take a look at our example eye prescription chart to apply your knowledge.
Eye Prescription Abbreviations
OD: Your right eye. OD stands for oculus dexter, which is “right eye” in Latin. We get the words dextrous and dexterity from the same root!
OS: Your left eye. OS stands for oculus sinister, which is “left eye” in Latin. We promise there’s nothing more morally sinister about your left eye than your right—in this case, the word is merely referring to direction.
OU: Both of your eyes. OU stands for oculus uterque, which is “both eyes” in Latin.
NV: Near vision (seeing things close-up).
DV: Distance vision (seeing things far away).
PD: Pupillary distance. Your pupillary distance is—drumroll—the distance between your pupils! It’s a handy measurement that helps align the center of the lens with the center of your pupils so that you have the most accurate vision possible. Your pupillary distance can be measured in a number of ways, including manually or with a specialized device called a pupillometer.
In an ideal world, your eye doctor will include your PD on your prescription every time. But not all doctors do this. If yours doesn’t, you can always use our handy tool to measure your pupillary distance online.
SPH: Sphere. In this case, “sphere” means that the correction for nearsightedness or farsightedness is spherical—as in, equal in all meridians of the eye. You’ll see numbers listed in the SPH column, and we’ll get to those next…
Eye Prescription Scale
Diopters: Any numbers you see in the “Sphere” or “Cyl” column of your eye prescription stand for diopters. A diopter is a unit of measurement, like a gram or an ounce. Instead of measuring mass, however, diopters measure the refractive power of a lens. You’ll see larger numbers (more optical power!) if your prescription is higher and lower numbers if your vision doesn’t need as much help from glasses or contacts.
It’s important to note that these numbers proceed in either direction on a number line—they can be positive or negative, based on the type of refractive error in your eye. If you don’t need glasses at all, you wouldn’t need any optical power from lenses: 0 diopters.
Negative Numbers: If you see a minus sign in front of a number, that indicates nearsightedness, or myopia. Someone with -3 diopters of nearsightedness, for example, might have trouble reading words on a chalkboard from far away.
Positive Numbers: If you see a plus sign in front of a number, that indicates diopters that will correct farsightedness, or hyperopia. Someone with +3 diopters of farsightedness, for example, might struggle to read text that’s very close to their face.
Other Terms on Your Eye Prescription
Depending on the kinds of lenses you need, you may see other terms on your eye prescription, including:
CYL or Cylinder: This term is only relevant to people with astigmatism, and refers to the lens power needed to address it. Astigmatism means your eye’s cornea or lens is not completely spherical. Therefore, patients with astigmatism need cylindrical rather than spherical vision correction. (If there’s no number associated with this term on your prescription, then you have no astigmatism to correct!)
Axis: Again, this is for people with astigmatism. It’s the number (from 1 to 180) on your prescription that determines the orientation of your astigmatism correction. Axis is measured in degrees, not diopters. The cylinder and the axis always go together—you can’t have one without the other!
Add: Additional magnification. If you have age-related presbyopia, you may have difficulty reading text close up, and can reserve a section of your glasses lenses for some added magnifying power. It’s sort of like having reading glasses built into your regular ones.
Prism: Sometimes, eyes don’t move in alignment with one another, resulting in symptoms such as double vision. (One disorder that causes eye misalignment is strabismus, commonly known as “crossed eyes.”)
To account for this condition, your doctor can add a prism to your lenses. The prism is placed in a certain position and orientation based on your prescription, which will also notate the direction of the prism’s thickest edge, or base.
BU: base up
BD: base down
BI: base in
BO: base out
The prism will also have its own refractive strength, measured in prism diopters.
Expiration Date: Your eye prescription probably won’t be the same forever—it’s important to have your eyes examined regularly to make sure they’re seeing as best as they possibly can.
Eye prescriptions are normally valid for a year or two before they expire (the exact time period can vary depending on state laws). After that, you’ll need to renew it by scheduling an eye exam.
If you’re still seeing clearly through your glasses, we can renew it for you! It only costs $15, and you can do it at home.
Here are two examples of eye prescriptions, one for someone without astigmatism, and one for someone with it. Can you spot the differences and decode the terms you’ve learned? If so, you’ve learned how to read a glasses prescription like a pro.
A prescription written for someone with nearsightedness.
A prescription written for someone with nearsightedness and astigmatism.
Is Your Eye Prescription “Bad”?
We get asked this question all the time: “How bad is my eye prescription?” But we don’t go in for that kind of negative self-talk! There’s no such thing as a “bad” eye prescription—you likely mean that you’re worried your prescription is unusually strong.
If you’re interested in cutoffs between moderate and severe kinds of vision issues, we can tell you that nearsightedness needing a refractive correction of -5 diopters or more is often considered “high myopia,” whereas a prescription reading +5.25 diopters fits the bill for “high hyperopia.” However, these scales can vary from institution to institution.
Another fun fact: people with severe nearsightedness can experience issues with their near vision in addition to their distance vision, and people with severe farsightedness can have their distance vision affected, too. Vision correction is complex!
The most important takeaway is that your eyesight isn’t “good” or “bad”—it just needs a specific amount of stylish tech to be as sharp as you want it to be.
Will Your Eye Prescription Change?
Large fluctuations in your vision prescription over time aren’t normal and should be investigated (unless they’re the anticipated result of a treatment or injury). But small, gradual changes can certainly occur, especially as you and your eyes age.
For example, you can expect to develop presbyopia as you get older (it typically happens to most of us in our forties). When that happens, you might want to get a pair of reading glasses or incorporate new progressive lenses into your regular frames.
Even if you don’t think your vision is any different and you’re not experiencing any worrisome symptoms, it’s a good idea to get your eyes checked every year. Annual eye exams ensure that an optometrist is monitoring your eye health and updating your prescription as needed.
Due for a check-up?
We know some pretty great doctors who’d love to meet you and your eyes.
You might be wondering: Is my contact lens prescription the same as my glasses prescription?
Not always! Your glasses sit at a slight distance away from your eyes, whereas contact lenses rest directly on your corneas. That difference in distance means their prescriptions might not match up.
Additionally, because contact lenses come with fixed parameters determined by their manufacturers, the doctor may need to adjust your contact prescription accordingly. These adjustments can also differentiate your contact prescription from your glasses prescription.
Even if the values on your contact lens and glasses prescription are the same, your contact prescription needs to include the name of the brand you’ve been prescribed (e.g. Acuvue, clariti, DAILIES, etc.).
Your optometrist also needs to put additional information on your contact lens prescription related to fit. That’s right—your contacts need to fit you, just like anything else you wear.
The two measurements you should also see on your contact lens prescription are:
Base Curve (BC): The curvature of your contact lens, which should mold comfortably to the curvature of your natural eye. This measurement in millimeters (mm) always falls between the numbers 8 and 10.
Diameter (Dia): Just like in regular math, the diameter is the width of a contact lens in millimeters. Most contact lenses are between 13 and 15 mm wide.
Without all of the information above, you won’t be able to purchase contact lenses. Submitting your glasses prescription alone won’t cut it! So, if you want to wear contacts in addition to (or instead of) glasses, be sure to let your eye doctor know when you schedule your exam. That way, they’ll know to include a contact lens fitting to assess the fit and vision of the contacts.
Remember: You Have a Right to Your Eye Prescription
According to the Federal Trade Commission, you are legally entitled to a copy of your eye prescription. Your eye doctor should provide you with one after your exam, or whenever you request it.
You don’t have to purchase eyewear from their office to get your eye prescription, and you should never have to pay a fee solely to access it. We recommend holding on to a copy so that it’s extra-easy to shop for glasses and contacts online.
Now that you understand your eye prescription perfectly, all that’s left is to find frames (or contacts) that make you feel fab!
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