How to read your eyeglass prescription.

An eyeglass prescription can look like a complex code. This guide deciphers every term on your eye prescription chart, so you can understand your vision needs.

The basics: right eye vs. left eye.

The first step is knowing which eye the numbers refer to. These abbreviations are derived from Latin and are standard across all prescriptions.

• OD (Oculus Dexter). This is the Latin term for the right eye. All parameters listed in the row or column labeled "OD" correspond to the corrective lens for your right eye.
• OS (Oculus Sinister). This is the Latin term for the left eye. All values associated with "OS" are for your left eye. It's common to see both od eye and os eye on a prescription.
• OU (Oculus Uterque). You may occasionally see "OU," which means "both eyes." This is used when a parameter is the same for both eyes.

Decoding your corrective power: SPH, CYL, and axis.

These values are the most critical components of your prescription, detailing the specific power your lenses need to correct your vision.

SPH (sphere): Correcting nearsightedness and farsightedness.

SPH is the main lens power required to correct your vision. It's measured in a unit called diopters, meaning refractive power.

• A minus sign (-) before the SPH number indicates correction for myopia (nearsightedness), meaning you have difficulty seeing distant objects clearly. The lens corrects this by modifying how light focuses on your retina.
• A plus sign (+) indicates correction for hyperopia (farsightedness), meaning you have difficulty seeing objects up close.

CYL (cylinder) and axis: correcting astigmatism.

If you have numbers in the cyl and axis columns, you have astigmatism. This is a common condition affecting about one in three people in the United States and is caused by an imperfectly curved cornea or lens. A non-astigmatic eye is shaped like a basketball (spherical), while an astigmatic eye is like an American football (toric). These two values always work together.

• CYL. This indicates the diopter power needed to correct the astigmatism.
• Axis. It's a number from 1 to 180 degrees that indicates the orientation of the astigmatism. This tells the lab where to place the cylinder correction on the lens.

Additional power for multifocal lenses and special corrections.

Some prescriptions include extra values for more specific vision needs.

ADD (added power) for presbyopia.

Add refers to the added magnifying power placed in the lower part of multifocal lenses (like bifocals or progressives). This value corrects presbyopia, the age-related difficulty with near vision that makes reading a challenge. It's almost always a positive number and is usually the same for both eyes.

Prism: correcting eye alignment.

Prism correction is included for people who experience diplopia (double vision), which can be caused by eye alignment issues. The prism bends light to correctly align the two images your eyes see into one single image. It's measured in prism diopters (p.d.) and includes a base direction: BU (Base Up), BD (Base Down), BI (Base In), or BO (Base Out).

Critical measurements for manufacturing your glasses.

This final measurement is essential for crafting lenses that are centered correctly for your eyes.

PD (pupillary distance): aligning your lenses.

The term PD on a glasses prescription stands for Pupillary Distance. It's the distance in millimeters between the centers of your pupils. This measurement is used to align the optical center of your lenses with your eyes. An accurate PD provides the clearest vision.

Putting it all together: a summary table and examples.

Here is a quick reference table to summarize the terms on your glasses prescription.

Conclusion.

Understanding your eyeglass prescription gives you more control over your eye care. By knowing what OD/OS, SPH, CYL, Axis, Add, and PD mean, you can confidently discuss your visual needs with an optician. After you receive an updated prescription, you can begin to explore different types of eyeglass lenses and coatings. Knowing your prescription is also the first step in learning how to choose the best glasses for your face shape and lifestyle, estimating the cost of prescription glasses, or when you decide to buy glasses and contacts online.

FAQs.