Understanding Your Glasses Prescription: What the Numbers Mean
When you leave your eye doctor's office and look at your prescription, it may seem like an unintelligible series of numbers and abbreviations. That code lets your eyeglass manufacturer know not only if you're nearsighted or farsighted, but also if you have an astigmatism or presbyopia.
This article will go behind the numbers to help you understand your prescription so you can be knowledgeable about your own eye health, track any changes in your vision, and talk comfortably with your eyecare practitioner. Because as we all know, knowledge is power.
When you look at a standard prescription, the first word you'll see is "Distance." This term refers to what power of lens you will need to see distant objects clearly. Your ophthalmologist or optometrist determines this power by trying different lenses while you read an eye chart in the exam. The strength of lens correction you need may be different for each eye.
Moving from left to right on the prescription pad, after "Distance" you see the letters OD and OS. These stand for right eye ("oculus dexter") and left eye ("oculus sinister") in Latin. You may just see RE for right eye and LE for left eye, as some doctors have moved away from the traditional Latin prescription terms. If you see OU, it stands for "oculus uterque," meaning both eyes measured the same.
Next comes "Sphere" or "SPH." This refers to the lens power you'll need to achieve 20/20 (or at least optimal) vision. A minus sign (-) means you are myopic, or nearsighted. A plus sign (+) means you are hyperopic, or farsighted. (If you see no + or - sign, it also means you are farsighted.) The higher your SPH number is, the more correction you will need.
In the past, high sphere numbers meant you would be prescribed "Coke-bottle" glasses, but today a good optician will work with you to minimize lens thickness. One recommendation is to use acetate frames with thick edges and thick temples to disguise the lens and reduce eyeglass weight. The new high-index plastic lenses will also help reduce weight and thickness. These will probably come with an anti-reflective coating because the higher the index, the more reflection and glare from the lens. And the anti-reflective coating minimizes the appearance of the lenses, an aesthetic plus.
If you have presbyopia and need multi-focal lenses (i.e. bi-focals, tri-focals, or progressives), you will see numbers in both the "D.V." (distance vision) and "N.V." (near vision) sections of your prescription. You may also see the word "Add" written below "Distance" on your prescription. The add number represents the magnifying power that will be added to the lower half of your progressive or bi-focal lenses to help restore your near vision. (No more having to hold menus at arm's length.) This number is typically the same for both eyes.
Look over at the next field on your prescription and you'll find the word "Cylinder," or "CYL." This is the lens power you'll need to correct astigmatism, which creates blurry vision at any distance. It is a common vision condition, usually present at birth, caused by an irregularly shaped cornea (the front cover of the eye) or irregular curvature of the lens inside the eye.
If the cylinder field is blank, you don't have astigmatism, or you only have a very mild one that doesn't need correcting. Again, a minus sign (-) corresponds to a nearsighted astigmatism, and plus sign (+) corresponds to a farsighted astigmatism.
Information about the "Axis," or "X," is in the adjacent field on your prescription. The axis is a number between 1-180 indicating the meridian of the eye. If you imagine your eye as a circle and draw a horizontal line to halve that circle, the meridian is one half. If you think back to high school geometry, you'll remember that "90" is the vertical line drawn straight up and down (the vertical meridian), while "180" is the horizontal line (the horizontal meridian).
If your prescription indicates that you need a cylindrical lens power (for astigmatism), then you'll also have an axis power. The axis number tells the eyeglass manufacturer where on your lens the astigmatism correction should be located.
The last two terms on the prescription are "Prism" and "Base." Some eyeglasses have a prism added because the person's eyes are not properly aligned. This is not a common problem, however, so oftentimes this field is left blank on your prescription.
The base indicates where the thickest edge of the prism will be: BU (for base up), BD (for base down), BI (for base in toward the nose), or BO (for base out toward the ears). The base direction tells you where the prism will redirect light.
The Vision Council estimates there are over 149 million adult eyeglass wearers in the United States (more than half of U.S. adults), with 90.5 million of them over the age of 44. With an aging population, this number is bound to increase.
It's time to arm ourselves with knowledge about our health, especially when it concerns one of our most precious organs, our eyes. That empowerment starts with understanding how to read your own eyeglass prescription.
As the Optician's Handbook puts it, "a prescription is an equation." Your optician can balance that equation to create the best possible pair of eyeglasses for your eyes, your face, and your lifestyle. And now you, too, can grasp what's behind those seemingly mysterious numbers.
Eye health is complete when you see clearly through a new pair of glasses. Now, moving from eye exam to holding a pair of prescription glasses in your hand is faster than ever.
With the Internet came the ease of ordering prescription glasses and sunglasses online. Plus, high-end prescription eyewear retailers like FramesDirect.com give you the added benefit of having certified opticians review your uploaded prescription before they place your order.
With so many choices of eyeglasses and quick ordering, whether you're indoors or out, playing sports or reading, you'll never be without clarity.
Sources: Optician's Handbook, AllAboutVision, The Vision Council, American Optometric Association