How to Read Eye Prescription Numbers
The first time you get a prescription for your eyes, it can be confusing. You probably start out with no idea how to read eye prescription numbers – plus, some of them seem to be different. Your eye doctor can help you get through your confusion, but here is an overview of both eyeglass and contact lens prescriptions.
When you go into the eye doctor, no matter what age you are on your first visit, you will walk out with a prescription – but not all prescriptions require correction. You could have 20/20 vision, which is “perfect” vision, or you could have only astigmatism or a small prescription that doesn’t need correcting, at least yet.
Your prescription can change for many reasons. Vision changes as you get older, if you suffer from major eye injuries or trauma, and can change if you get an eye disease, like Glaucoma. This is why eyeglass prescriptions are only good for one or two years, and contact lens prescriptions expire after one year.
Eyeglass and Contact Lens Prescriptions
An eyeglass prescription is the reading that will be used to make corrective lenses that will help you see better. It’s important to realize that even with correction, you might not have 20/20 vision. A contact lens prescription does more than correct your vision – It also includes the measurements of your eye so that you get the right size of contact to wear.
Your eye doctor does an exam that looks at what type of correction you need in order to see better – some people have an issue seeing things far away, while others can’t see things close-up. As you get older, you may have issues that combine nearsighted and farsighted vision, which requires bifocals or even trifocals.
At the end of your exam, you will walk out of the doctor’s office with a written prescription, either for eyeglasses or contact lenses (or both). Both of these prescriptions are different and require a different exam.
Who Writes Your Prescriptions and Who Fills Them?
Prescriptions for your corrective lenses will be written by an optometrist or an ophthalmologist. These are the doctors that have gone through years of school to detect eye diseases and to be able to make it so you can see better. Once they give you your prescription, you will then work with a dispensing optician who will help you find the right frames, pick the right lenses, and ensure that your glasses fit your face.
If you’re getting contacts, you will work with an optician that has been trained in contact fitting and care before you walk out with a new type of contact lens for the first time.
How Eye Doctors Come Up With Your Prescription
All of those random lenses the doctor has you look through, “Is “a” better than “b”?”, are telling your eye doctor which prescription lenses will work best for you when it comes to eyeglasses. Your prescription will include the lens power that is needed in order to correct your vision, and it will also include information on whether or not you have any prism (which helps correct double vision), and what your pupillary distance is – all of these things are important in determining the lens prescription you need for eyeglasses.
Your actual prescription is most often determined when your doctor does the refraction, which uses a phoropter (where you pick between those two lenses).
Contact lens prescriptions are a bit different. They have more to them because of the measurements of your actual eye, and they will also have different “powers” than your eyeglass prescription, since they will be correcting vision right on your eyes, instead of from 12 millimeters in front of them.
Reading Your Eyeglasses Prescription
Eyeglass prescriptions include OS and OD numbers. These are Latin abbreviations for oculus sinister (the left eye) and oculus dextrus (the right eye). If your prescription says OU on it anywhere, it is referring to both eyes.
The easiest thing to remember when it comes to determining how “intense” your prescription is, is knowing that the higher the numbers, the worse your prescription. Then, when the numbers have a plus sign, this is an indication of farsightedness (more professionally known as hyperopia). A minus sign shows nearsightedness, or myopia.
The numbers on your prescription are representing diopters, which is the measure of correction for your eyes and is also known as your focusing power. Diopter may be referred to on your prescription as only a “D.”
If your eyes, or just one of them, has a difference in normal curvature, that means you have astigmatism. This reading will be recorded with an Axis number.
It will also have an “S,” for spherical (giving info on the degree of nearsightedness or farsightedness) – this is also referred to as the “power,” especially when it comes to contact prescriptions. , and a “C.” The “C” is for cylinder, which is the degree of astigmatism.
Your optician will measure the distance between your pupils with a pupilometer, which is an important measurement for making sure that the center of your lenses is in the right place on each eye.
Reading Your Contact Lens Prescription
Reading your prescription for contact lenses is different than that for eyeglasses, but you will see some familiar terms – like OS, OD, and OU. If your contact lens prescription is the same in both eyes (which does happen, surprisingly), OU is used. Cylinder and Axis will both be there as well if you have astigmatism.
Not only are there more things to read on your contact prescription, but you will also find that the prescription numbers will be different. Using an eyeglass prescription for contacts would make your contacts way too strong.
Here are some of the other things you’ll find on your contact lens prescription, which differ from eyeglasses (these are all important features that ensure your contact lenses fit properly and help you see better) –
The diameter measurement, or dia, is the diameter of the contact lens, which goes by the diameter of your eye. This is a width measurement that is recorded in millimeters. Contact lenses usually average around 13.5 to 15 millimeters.
2. Base Curve
The base curve is another contact lens-only measurement that is referred to as BC on prescriptions. This is the measurement of the curve of the inside of a contact lens – a measurement that is important to ensure the lens curves with your eye. This is also measured in millimeters.
The base curve is most often somewhere between 8 and 10. The right bc measurement ensures that the contact lenses fit properly and comfortably.
3. Add Power
If you need multifocal lenses (yes, they do make those in contact lenses), your prescription will have an added power on it. The plus sign for the add power is indicating the additional power that is needed to give the wearer clearer vision at close range – which means it is also used for people that only have presbyopia as well (think of it like the numbers you see on reading glasses).
Color will only be included on your prescription if you’re opting to get prescription lenses that will also change the color of your eyes. You might think of these as merely cosmetic lenses, but you can get them in prescriptions.
Colored lenses come in a couple of options (not to be mistaken with the Halloween style lenses). There are lenses that are meant to only enhance your natural eye color, and there are opaque contacts that will completely change your eye color. Want to make your brown eyes blue? Get opaque contacts in blue.
Your prescription will also include the brand name of the contacts you’ve been prescribed. Companies make their lenses different, and the brand you’re prescribed may be that way because they are the ones that have your specific prescription or make lenses that fit your eyes most comfortably.
The company that makes them will be listed, like Acuvue. And then it will also include the make of the lens.
Final Thoughts on How to Read a Prescription
You need to know how to read an eye prescription so that you can ensure your prescription is correct and contains all of the information required prior to leaving your doctors office. This is especially important if you will be ordering your glasses or contacts elsewhere.