How to Read your Prescription, By Gary Heiting, OD

So, you’ve just had an eye exam and your optometrist or ophthalmologist has given you an eyeglass prescription. He or she probably mentioned that you are nearsighted or farsighted, or perhaps that you have astigmatism.

But what do all those numbers on your eyeglass prescription mean? And what about all those abbreviated terms, such as OD, OS, SPH and CYL?

This article will help you decipher all parts of your prescription and discuss it knowledgeably with an optician when you’re buying eyeglasses.

What OD and OS Mean

The first step to understanding your eyeglass prescription is knowing what “OD” and OS” mean. They are abbreviations for oculus dexter and oculus sinister, which are Latin terms for right eye and left eye.

Your eyeglass prescription also may have a column labeled “OU.” This is the abbreviation for the Latin term oculus uterque, which means “both eyes.”

Though the use of these abbreviated Latin terms is traditional for prescriptions written for eyeglasses, contact lenses and eye medicines, some doctors and clinics have opted to modernize their prescriptions and use RE (right eye) and LE (left eye) instead of OD and OS.

You may have noticed that on your prescription form the information for the right eye (OD) comes before the information for the left eye (OS). Eye doctors write prescriptions this way to avoid making errors, because when they face you, they see your right eye at left (first) and your left eye at right (second).

Other Terms on Your Eyeglass Prescription

Your eyeglass prescription contains other terms and abbreviations as well. These include:

Sphere (SPH). This indicates the amount of lens power, measured in diopters (D), prescribed to correct nearsightedness or farsightedness. If the number appearing under this heading has a minus sign (–), you are nearsighted; if the number has a plus sign (+) or is not preceded by a plus sign or a minus sign, you are farsighted.

The term “sphere” means that the correction for nearsightedness or farsightedness is “spherical,” or equal in all meridians of the eye.

Cylinder (CYL). This indicates the amount of lens power for astigmatism. If nothing appears in this column, you have no astigmatism.

The term “cylinder” means that this lens power added to correct astigmatism is not spherical, but instead is shaped so one meridian has no added curvature, and the meridian perpendicular to this “no added power” meridian contains the maximum power and lens curvature to correct astigmatism.

The number in the cylinder column may be preceded with a minus sign (for the correction of nearsighted astigmatism) or a plus sign (for farsighted astigmatism). Cylinder power always follows sphere power in an eyeglass prescription.