How to Read an Eyeglasses Prescription

Posted on August 28, 2024 in Vision Health

When was the last time you really looked at your eye prescription for your glasses? If you’ve ever felt overwhelmed by the assortment of letters and numbers on that small slip of paper, you’re not alone.

Deciphering these codes can initially seem daunting, but once you understand what these abbreviations and numbers mean, reading your eye prescription can be fairly simple!

Understanding the basics of an eye prescription

Your eye prescription is a written order from your eye doctor that specifies the correction needed for your eyesight. It typically includes several key elements, each represented by specific abbreviations and numbers. 

Here’s a breakdown of the common components:

• OD (Oculus Dexter): Refers to the right eye.
• OS (Oculus Sinister): Refers to the left eye.
• OU (Oculus Uterque): Refers to both eyes.

What do the numbers mean in my eye prescription? 

Using a series of tests, your eye doctor will determine what strength of the prescription lens is needed to correct your eyesight. These numbers can be positive (+) or negative (-), ranging from -5 to +3.

A negative number means that the prescription is correcting nearsightedness, or myopia, with -5 being severe and -1 being more minimal. A positive number indicates farsightedness. The higher the number, the more correction is needed.

These numbers associated with each eye can be the same or different, and it's not uncommon for the prescriptions to differ between each eye.

Age plays a factor, of course. Two to four percent of children are diagnosed with differing powered lenses, a condition known as anisometropia. This condition becomes more prevalent as you get older, affecting 10 percent of individuals under 65 and 25 percent of those over 85. 

Comparing SPH and CYL numbers

When you receive your eye prescription, you may also notice numbers next to the abbreviations SPH and CYL. These numbers represent the shape of your eyes and help determine the type of correction needed.

• SPH (Sphere) - Most eyes have a round shape. If your eyes are the standard round shape, the positive or negative number next to SPH is the only prescription number needed to correct your vision.
• CYL (Cylinder) - About one-third of Americans have oval-shaped eyes. This unique shape causes a refractive error known as astigmatism, where light cannot properly enter the retina. If you don't have astigmatism, you will only see numbers next to the letters SPH in your prescription. However, if your eye's curvature needs correction, your eye doctor will also enter a number in the CYL column.

Understanding AXIS and ADD in your eye prescription

If you have astigmatism, you'll also see an AXIS value, measured in degrees, in your eye prescription. This number will range from 1 to 180 degrees and indicates where the astigmatism is located on the cornea. 

Additionally, for those who need bifocals or progressive lenses, an ADD value may be included, specifying the additional magnification required for the bottom of the lens.

Example of an eye prescription

Let’s walk through an example of what a typical eye prescription might look like:

• OD (Right Eye): SPH -2.50, CYL -1.00, AXIS 180
• OS (Left Eye): SPH -3.00, CYL -0.50, AXIS 90
• ADD +1.50

In this example:

Note: A diopter is a measurement that tells you how strong a lens is and what type of correction it provides for your vision.

• For the right eye (OD), the prescription indicates that you are nearsighted with a sphere value of -2.50 diopters. You also have astigmatism, requiring a cylindrical correction of -1.00 diopters at an axis of 180 degrees.
• For the left eye (OS), you are also nearsighted with a sphere value of -3.00 diopters and have astigmatism with a cylinder of -0.50 diopters at an axis of 90 degrees.
• The ADD value of +1.50 diopters indicates additional magnification needed for reading or close-up tasks.

If you feel overwhelmed by this example, don’t worry! You can always ask your provider for a more in-depth explanation of your eye prescription. 

Feel confident in your eye care

Understanding the various numbers and abbreviations in your eye prescription can empower you to take an active role in your eye care. The next time you receive your eye prescription, you'll be able to read it with confidence, knowing exactly what each part means for your vision.

Pro tip: Having vision insurance makes it easy to protect your eye health with regular comprehensive eye exams. Learn more about our DeltaVision plans.