Ever wonder what the numbers mean in your glasses or contact prescription? You’re in luck! I will review all the essential details in this post so you can be informed when you go to the eye doctor. I will also discuss how you can get these numbers to ZERO – as in no prescription!
First, let’s start with some terminology you need to know to decode a glasses or contact prescription.
Quantifies the imperfection in how your eye focuses images onto your retina. You can liken the cornea and lens of your eye to the lens of a camera and the retina to the film of a camera. The job of your cornea and lens, then, is to bend light perfectly so images land clearly on your retina.
Most patients have some refractive error, meaning they need either contacts, glasses, or surgery to achieve perfect vision. The three most important types of refractive error are myopia, hyperopia, and astigmatism (discussed below).
MYOPIA (a.k.a nearsightedness)
Your eye brings images into focus in front of your retina. By the time the light reaches your retina, the image is out of focus. Myopia is much more common than hyperopia, and most patients seeking laser eye surgery are myopic.
Myopic patients can see up close but cannot see far away. The classic myopic patient is the one who struggles to read the chalkboard in school, prompting her parents to bring her to the eye doctor to be fitted for glasses.
HYPEROPIA (a.k.a farsightedness)
Your eye brings images into focus behind your eye, meaning they are out of focus when they cross your retina. Many hyperopic patients do not need distance glasses until their thirties or forties because they correct their own hyperopia by accommodating. When they can no longer accommodate enough, farsighted patients have blurry vision at both distance and near!
The process by which the lens inside our eye changes shape to bend light more. More bending adds power to your eye so you can read up close.
Try this test at home: place your finger about two feet in front of your face. When you focus in the distance, your finger is blurry. When you focus on your finger, you are accommodating. This makes your finger look clear and distance look blurry. As we age, we slowly lose the ability to accommodate. This is why we need reading glasses, usually starting in our forties.
Your eye bends light differently depending on which axis of your eye the light passes through. This happens when your cornea or lens is aspheric (more football-shaped than basketball-shaped). Both hyperopic and myopic eyes can have astigmatism.
What about an eye that has one farsighted axis and one nearsighted axis? That is called MIXED ASTIGMATISM.
**Laser eye surgery can treat ALL of these refractive errors, often more accurately than contacts and glasses.**
O.D. is Latin for Ocular Dexter and means right eye.
O.S. is Latin for Ocular Sinister and means left eye. In ancient times, left-handed people were considered evil or possessed – hence the term sinister.
OK, enough definitions. Let’s look at a sample glasses prescription. Note that there are separate rows for O.D. and O.S. for the common occurrence of your right and left eyes having different refractive errors.
The first column is the SPHERE, which represents the base power of your eye. A PLUS symbol means farsighted, and a MINUS symbol means nearsighted.
The second number is the CYLINDER, which measures the amount of astigmatism. The units of sphere and cylinder are both called diopters.
The third number is the AXIS, which describes the location of the astigmatism. The units are degrees, from 0° (which is the same as 180°) to 179°.
PRISM is for patients with double vision who need special glasses to see single again. We’ll ignore prism for this discussion.
ADD is for patients who need additional power in the bottom portion of their glasses to read up close, either in bifocals or progressive lenses.
P.D. (not on this sample form) stands for pupillary distance. P.D. describes how far apart the optical centers of the lenses must be to match up with the centers of your pupils. Usually, this number is somewhere between 55 and 70 millimeters.
Take the sample right eye prescription: -2.00 – 0.75 x 90. This eye has a base myopic prescription of -2.00 and an additional 0.75 diopters of myopia at the 90° axis. (Read last week’s post to discover why this is an excellent prescription for SMILE).
OK, but what about contacts? My contact script has different boxes than my glasses script, and the numbers don’t even match up! What’s the deal? Let’s examine this example script to learn more.
As you can see, the contact script still has the sphere, axis, cylinder, and add. (Note: this picture shows a different prescription than the glasses script above). But we also see B.C., DIA, and Brand. Brand is self-explanatory. This box allows the doctor to indicate the brand of lenses best suited for you. B.C. stands for base curve and describes the curvature of the contact lens. The steepness of your cornea will determine the ideal base curvature of contact lens to optimize fit and comfort. Finally, DIA is for diameter, which, as you may surmise, describes the diameter of the contact lens. Altering the diameter of the lens can also fine-tune fit and comfort.
Sometimes, the sphere and cylinder will be slightly different in a contact prescription versus a glasses script. This is because contacts are closer to your eye (they literally touch your cornea!), so they must bend light a little more strongly to land it clearly on your retina, compared to glasses lenses that sit about 13 millimeters from your corneas. Thus, the magnitude of the sphere and cylinder may be slightly more positive (or less negative) in a contact prescription. The axis, however, is always the same.
You might be a math nerd like me if you’re still reading. There is a lot of math in optics and laser eye surgery, which is one of the reasons I love my profession so much. Regardless of your feelings about math – new or old – there is one opinion we can all agree on: A prescription of 0.00 – 0.00 x 000 is pretty awesome. This describes an eye with zero refractive error! Unless they hit the genetic jackpot, this prescription is likely from an eye that underwent laser eye surgery. Laser eye surgery reshapes the cornea so that it can focus images perfectly on your retina without refractive error.
Keep reading our weekly blog and follow us @corsinilasereye to learn more about optics, eye health, laser eye surgery, and more. Interested in a zeroed-out prescription? Call or request a free, no-hassle appointment to learn your options.
Thanks for calculating with me!
Jonathan Corsini, MD