Eye Prescriptions (Meaning of SPH, CYL, AXIS)

What Do the Abbreviations on My Glasses Prescription Mean?

During an eye exam, your doctor will determine whether you need vision correction. They may mention that you have either of the following:

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

These are all crucial pieces of information when getting 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. By the end of this article, you’ll know exactly how to read your glasses prescription and what your prescription means.

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 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.

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

How to tell eyeglass prescription — Meridians of the eye are imaginary lines that pass through the pupil's center, measured by a protractor superimposed over the eye's surface. The 90-degree meridian is vertical, and the 180-degree meridian is horizontal.

It is measured in diopters, the unit of measurement for the lens's refractive (light-bending) power.

If the number is negative (–), it means it is correcting nearsightedness. If the number is positive (+), it is correcting farsightedness.

A number between +/-.025 to +/-2.00 is considered mild, a number between +/-2.25 to +/- 5.00 is considered moderate, and a number greater than +/- 5.00 is considered severe.

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. A minus sign (–) means it's correcting nearsighted astigmatism, while plus sign(+) means it's 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 with 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, which 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 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.