How to read your eyeglasses prescription
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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 read and understand your eyeglass prescription so you can discuss it with an optician when you buy new 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 use of these abbreviated Latin terms is common on prescriptions for glasses, contact lenses and eye medicines, some doctors and clinics have opted to modernize their eyeglass prescriptions and use RE (right eye) and LE (left eye) instead of OD and OS.
On your eyeglasses prescription, the information for your right eye (OD) comes before the information for your left eye (OS). Eye doctors write prescriptions this way because when they face you, they see your right eye on their left (first) and your left eye on their right (second).
Other parts of your eyeglasses 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 it has a plus sign (+), you are farsighted.
Cylinder (CYL). This indicates the amount of lens power for astigmatism. If nothing appears in this column, you have little or no astigmatism that requires correction.
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 correction of nearsighted astigmatism) or a plus sign (for farsighted astigmatism). Cylinder power always follows the sphere power in an eyeglass prescription.
Meridians of the eye are determined by superimposing a protractor scale on the eye's front surface. The 90-degree meridian is the vertical meridian of the eye, and the 180-degree meridian is the horizontal meridian.
Axis. This describes the lens meridian that contains no cylinder power to correct astigmatism. The axis is defined with a number from 1 to 180. The number 90 corresponds to the vertical meridian of the eye, and the number 180 corresponds to the horizontal meridian.
If an eyeglass prescription includes cylinder power, it also must include an axis value, which follows the cyl power and is preceded by an "x" when written freehand.
The axis is the lens meridian that is 90 degrees away from the meridian that contains the cylinder power for astigmatism correction.
Add. This is the added magnifying power applied to the bottom part of multifocal lenses to correct presbyopia. The number appearing in this section of the prescription is always a "plus" power, even if it is not preceded by a plus sign. Generally, it will range from +0.75 to +3.00 D and will be the same power for both eyes.
Prism. This is the amount of prismatic power, measured in prism diopters ("p.d." or a superscript triangle when written freehand), prescribed to compensate for eye alignment problems. Only a small percentage of eyeglass prescriptions include prism.
When present, the amount of prism is indicated in either metric or fractional English units (0.5 or ½, for example), and the direction of the prism is indicated by noting the relative position of its "base" (thickest edge). Four abbreviations are used for prism direction: BU = base up; BD = base down; BI = base in (toward the wearer's nose); BO = base out (toward the wearer's ear).
Sphere power, cylinder power and add power always appear in diopters. They are in decimal form and generally are written in quarter-diopter (0.25 D) increments.
Axis values are whole numbers from 1 to 180 and signify only a meridional location, not a power. When prism diopters are indicated in decimal form, typically only one digit appears after the period (e.g., 0.5).
Additional Information. Your eye doctor also might write specific lens recommendations on your eyeglass prescription — such as anti-reflective coating, photochromic lenses and/or progressive lenses — to give you the most comfortable vision correction possible.
An example of an eyeglass prescription
Confused? Let's use an example to clear things up. (Yes, pun intended.)
Here is a sample eyeglass prescription:
OD -2.00 SPH +2.00 add 0.5 p.d. BD
OS -1.00 -0.50 x 180 +2.00 add 0.5 p.d. BU
In this case, the eye doctor has prescribed -2.00 D sphere for the correction of myopia in the right eye (OD). There is no astigmatism correction for this eye, so no cylinder power or axis is noted. This doctor has elected to add "SPH," to confirm the right eye is being prescribed only spherical power. (Some doctors will add "DS" for "diopters sphere;" others will leave this area blank.)
The left eye (OS) is being prescribed -1.00 D sphere for myopia and -0.50 D cylinder for the correction of astigmatism. The cyl power has its axis at the 180 meridian, meaning the horizontal (180-degree) meridian of the eye has no added power for astigmatism and the vertical (90-degree) meridian gets the added -0.50 D.
Both eyes are being prescribed an "add power" of +2.00 D for the correction of presbyopia.
This eyeglass prescription also includes prismatic correction of 0.5 prism diopter in each eye. In the right eye, the prism is base down (BD); in the left eye, it's base up (BU).
An eyeglass prescription cannot be used to buy contact lenses
Eyeglass and contact lens prescriptions aren't the same. An eyeglass prescription is for the purchase of eyeglasses only. It does not contain certain information that is crucial to a contact lens prescription and that can be obtained only during a contact lens consultation and fitting.
Eyeglass lenses are positioned at a distance from the eyes, while contacts rest directly on the eyes. That distance affects the lens power required for eyes to focus properly.
In addition to the information in an eyeglass prescription, a contact lens prescription must specify the base (central) curve of the back surface of the contact lens, the lens diameter, and the specific manufacturer and brand name of the lens.
Also, the power of an eyeglass prescription frequently is modified when determining the best contact lens power. This is because eyeglass lenses are worn some distance (usually about 12 millimeters) from the surface of the eye, whereas contact lenses rest directly on the cornea of the eye.
An accurate contact lens prescription can be written only after a contact lens fitting has been performed and the prescribing doctor has evaluated your eyes' response to the lenses and to contact lens wear in general.
Your eyeglass prescription: It's yours to keep
The Federal Trade Commission (FTC) is the U.S. government's consumer protection agency. In 1980, the FTC's Prescription Release Rule became law. This rule requires eye doctors to give patients a copy of their eyeglass prescription at the end of an eye exam that includes a refraction.
The Prescription Release Rule is intended to allow the "portability" of your eyeglass prescription so you have the ability to buy glasses from the vendor of your choice.
Your eye doctor must give you a copy of the prescription whether or not you ask for it. Eye doctors may not condition the release of your prescription on your agreement to purchase eyeglasses from them, nor may they charge you an extra fee to release your prescription. They also may not disclaim liability for the accuracy of the prescription if you purchase eyeglasses elsewhere.
Eye doctors who violate the provisions of the Prescription Release Rule are subject to a civil penalty of $10,000 per violation.
If you feel your eye doctor has violated the rule or you want free information on other consumer issues, you can contact the FTC at 1-877-FTC-HELP.
Page updated July 2019