How to Read an Eyeglass Prescription
So you need glasses. Welcome to the club. According to the Vision Council of America, just about 75 per cent of adults need either glasses or contacts to correct their vision. About 64 per cent wear eyeglasses exclusively. Even those who wear contacts occasionally wear eyeglasses.
You hold the prescription in your hands and when you take a look at that little piece of paper, you see a mixture of numbers and letters that make absolutely no sense to you. They are, in fact, instructions to the optician who will grind your lenses into the perfect shape and thickness that will correct your vision problem.
Remember, we need glasses because the light from images of the people and things in our environment is not falling on exactly the right place on our cornea. If it doesn’t fall exactly right, we will not see things clearly. So all the numbers in our eyeglass prescription tell the optician who will grind your lens, exactly how to bend the light and guide it so it will fall in just the right place.
You trust the opticians know what they’re doing, but you can’t help thinking how nice it would be to take a little peek into the valuable skill they will put to use for you.
Your prescription will most likely have the same information on it as the sample below. Of course the numbers will be different and will reflect the specific needs you have to correct your vision. But the basics remain the same.
Numbers and letters
Many eye doctors and optometrists still use Latin abbreviations when writing their prescriptions so you will notice the letters OS and OD. These mean oculus sinister, which denotes the left eye, and OD means oculus dexter, right eye. However, if your eye specialist uses English instead, you will see RE and LE for right eye and left eye.
Since you have two eyes and each one is usually different from the other, you will have a specific prescription for each eye. Rarely will both eyes require the same correction, but if that happens, you’ll see the letters OU, which is Latin for oculus uterque. The English equivalent is BE, or both eyes.
SPH means Sphere. The numbers you see here are measured in units called diopters. Deviations from 0 may indicate some degree of astigmatism. All that indicates is that your cornea is not perfectly round.
This is a very common condition, but because the shape is a little less than perfect, the light coming into the eye doesn’t fall on exactly the right place for the best focus. Thus, you need glasses to guide the light to the right place.
Diopters indicate to the person grinding your lenses the optical power needed to make the necessary correction for farsightedness (hypermetropia) or nearsightedness (myopia). If this number begins with a minus sign (-), then you need a correction for nearsightedness. If it begins with a plus sign (+), you need a correction for farsightedness.
Generally, the farther away from 0 that number happens to fall, either on the plus or the minus side, the more vision correction you need. So if your correction happens to be -1.00, as in the sample, you have 1 diopter of nearsightedness and need a small amount of correction. However, a correction of -4.00 means your eye needs lots of help.
Cylinder or CYL records the amount of astigmatism you have. Again, the higher the number, the more correction you need. The optician knows exactly how to grind the lens to guide the incoming light to the right place for focus.
AXI or X tells how irregular the shape of the cornea is as well as the direction of that irregularity. That number tells the optician whether to place more thickness in the center of the lens or on the edges. Remember, this is all about focusing light on the right place on your cornea for you to see properly.
ADD directs the optician regarding how much power to add to the distance prescription. You can think of this as magnification. The larger things are, the easier they are to see. So if the eye perceives something to be larger, it will make it easier to focus on it.
PD is the measurement between your right pupil and left pupil in millimeters. The optician needs this measurement to know exactly where to place the focal point in your lenses as they sit in your new frames. The focal point, that area on the lens that funnels all the points of light into your pupil and onto your cornea so you can focus, must be at just the right location.
Down to Basics
At the end of your doctor’s appointment, instead of just leaving with your prescription, you should probably take a look at it. Look it over and ask any questions you might have. If you have astigmatism, you might ask the doctor to explain that to you. If you don’t know what it is, the mere sound of it could be scary.
If this is not your first time needing glasses, ask how this new prescription compares with your old one. If there’s a huge change over a short period of time, could this be a cause for concern?
If you have any other questions or concerns, this is of course the time to ask and discuss it with your doctor. Ask for an explanation if you need to. Don’t feel that any question is too odd or may sound too elementary. Your doctor understands that you have no training in this area and knows you need to depend upon their expertise.
Now that you know the basics of reading your eyeglass prescription, you’ll have a better idea of just what kind of correction the doctor has prescribed for you. You’ll also have a better understanding of the kinds of things the optician will do for you when grinding your lenses.
And of course your doctor will keep records of your prescriptions from year to year, but it’s also a good idea to keep a copy for yourself. Keep it with papers you take with you when you travel in case you lose or damage your glasses while out of town. At least having the prescription with you will help you replace them ASAP. If you’re like most people, you can’t go for very long without them.