Here's How to Read Your Eyeglass Prescription

Regular eye exams are an important part of keeping your eyes healthy and your vision sharp. At an eye exam, your optometrist will do a series of tests to evaluate your vision and check for any signs of eye disease. If your vision needs to be corrected, you’ll be given a prescription for eyeglasses, which can be hard to decipher. Here’s everything you need to make sense of the assortment of abbreviations and numbers included in your eye prescription.

Right Eye and Left Eye

Optometrists use the abbreviations “OD” and “OS” to denote your right and left eyes.

• OD is your right eye. It’s short for oculus dexter, the Latin phrase for “right eye”.
• OS is your left eye. It’s short for oculus sinister, Latin for “left eye”.
• OU is both eyes. Your vision prescription may also have a column labeled “OU”, the abbreviation for oculus uterque, Latin for “both eyes”.

Note that some doctors and clinics have opted to modernize their eye prescriptions by using RE (right eye) and LE (left eye) instead of OD and OS. The information for your right eye (OD or RE) always comes before the information for your left eye (OS or LE). This is because when your eye doctor faces you, they see your right eye first on their left and your left eye second on their right.

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Sphere (SPH)

The sphere column is often abbreviated as SPH and indicates the amount of lens power prescribed to correct nearsightedness or farsightedness.

• A minus sign (-) next to the number means nearsightedness (you see better up close and need distance correction).
• A plus sign (+) means farsightedness (you see better far away and need near correction).

Lens power is measured in diopters (D). The further away you get from zero on either the minus or plus side, the stronger your prescription is.

Cylinder (CYL)

The cylinder number is how much astigmatism you have, if any. Astigmatism is when the front part of your eye is shaped more like a rugby ball than a regular sphere.

The number in the cylinder column may have a minus sign (for correction of nearsighted astigmatism) or a plus sign (for farsighted astigmatism). If this column is empty, you either don’t have astigmatism or the degree of it is so small that it doesn’t need to be corrected.

Axis

If an eyeglass prescription includes cylinder power, it also needs to include an axis value, as this tells you where the astigmatism is on the cornea. The axis is defined with a number from 1 to 180. Imagine a protractor scale held up right in front of your eye—this is how meridians of the eye are determined.

• The number 90 corresponds to the vertical meridian of the eye.
• The number 180 corresponds to the horizontal meridian of the eye.

Add

Add is the magnifying power applied to the bottom part of multifocal lenses to correct presbyopia—the natural farsightedness that happens with age. Simply put, this additional magnifying power makes it easier for you to read. The number appearing in this column of the prescription is always a plus power, even when you don’t see a plus sign in front of the number. Generally, it will range from +0.75 to +3.00 D and will be the same power for both eyes.

Prism

Prism is used if you have double vision, which means that you see two images of one object. As this is not very common, this box is often left empty. If it’s filled in, the prism is usually written in fractions (for instance: 1 ½). Four abbreviations are used for prism direction:

• BU: base up
• BD: base down
• BI: base in (towards your nose)
• BO: base out (towards your ear)

These abbreviations tell the eyeglass manufacturer exactly where to position the prism on eyeglasses that correct double vision.

Special Details on Your Prescription

You may find additional information on your glasses prescription such as the recommendation of certain types of lens coating or special comments from your optometrist. The best thing to do is to take the prescription with you when you’re buying a new pair of glasses, so optometrists have an easier job in helping you to find your perfect pair of glasses.

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The Bottom Line

The abbreviations and numbers on your eyeglass prescription tell the eyeglass manufacturer what type of lenses you need and how strong they need to be. This prescription also informs them about the degree of nearsightedness, farsightedness, or astigmatism in each of your eyes.

As your vision can change over time, it’s important to see an optometrist annually, or at least every couple of years to protect your eye health. For your convenience, you can find us at eight different locations in and around Chicago. We also offer the largest selection of premium and handmade eyewear, and we’re happy to help you choose a pair of glasses that really suits your needs. Book your appointment now!