How to Read Your Eyeglasses Prescription

If you’ve ever had an eye exam, and your doctor has determined that you will need vision correction, they may have mentioned that you have astigmatism, nearsightedness (myopia), farsightedness (hyperopia), or another type of eye condition that affects your vision.

They probably gave you an eyeglasses prescription that looks something like this:

These are all crucial pieces of information when it comes time to get your eyewear. (Note that this is different than a contact lens prescription). But what exactly do all these abbreviations and numbers mean?

We’ll break down your prescription step-by-step so that you’ll know exactly how to read your glasses prescription, and what your prescription means by the end of this article.

What Do OD & OS Mean?

You’ll typically find the abbreviates OD and OS on the left side of your prescription. These are abbreviations for the Latin terms oculus dexter and oculus sinister. Essentially, OD means right eye, and OS means left eye. You may also have a column labeled OU, which means oculus uterque, or both eyes.

Some eye doctors will have modernized this and replaced OD with RE (right eye) and OS with LE. 

What Does SPH mean?

Sphere, sometimes abbreviated as SPH, indicates the amount of lens power needed to correct nearsightedness or farsightedness. It is measured in diopters, which is the unit of measurement for the refractive (light bending) power of a lens. If the number is negative (–), it means it is correcting nearsightedness. If the number is positive (+) it means it is correcting farsightedness.

The term “sphere” refers to the fact that distance vision correction is spherical – or equal in all meridians of the eye.

What Does CYL Mean?

Cylinder, abbreviated CYL, follows sphere power in your eyeglass prescription. It indicates the amount of lens power needed to correct astigmatism, measured in diopters. If this column is blank, it means you have no astigmatism. If the number contains a minus sign (–), it means it is correcting nearsighted astigmatism. If the number has a plus sign or is positive (+), it means it is correcting farsighted astigmatism.

Astigmatism correction is not spherical. Cylindrical correction means that one meridian has no added power, while the meridian perpendicular to it has the maximum lens curvature and power added to it.

What Does Axis Mean?

Axis refers to the lens meridian that contains no cylindrical power to correct astigmatism. The axis is labeled as a number from 1 to 180. If a prescription includes cylinder power, it must be followed by an axis. The axis value is often preceded by an “x” when written in freehand. 

The axis is always exactly 90-degrees away from the meridian that contains all of the correcting lens power for astigmatism.

What Does Add Mean?

Add indicates the magnifying power applied to the bottom part of bifocal, multifocal, or progressive lenses used to correct presbyopia (age-related farsightedness). This number is always positive, even if it doesn’t have a plus sign (+) in front of it. These numbers typically are the same for both eyes and range from +0.75 to +3.00.

What Does Prism Mean?

Prism refers to the amount of prismatic power needed to correct eye alignment problems, measured in prism diopters. This number is followed by the base direction, or thickest edge, of the prism. This is only present if your eyes do not focus well together as a pair, such as in strabismus (eye turn).

This number will appear as a decimal (0.5) or fraction (½) followed by two letters indicating the base direction:

• BU stands for "base up."
• BD stands for "base down."
• BI stands for "base in."
• BO stands for "base out."

Additional Eye Prescription Information

There may be additional information written in by your optometrist or ophthalmologist regarding your eye health. These will usually be specific types of corrective lens recommendations such as multifocal lenses, progressives, anti-reflective coating, photochromatic, or transition lenses. Some eyeglass prescriptions may require a PD (pupillary distance) measurement.

Remember, eyeglasses prescriptions are different than prescriptions for contact lenses. A contact lens prescription can only be obtained from a contact lens consultation and fitting.

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