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How to read your eyeglasses prescription

Eyeglass prescription example

You've just had an eye exam and your optician has given you a spectacles prescription.

He or she probably mentioned that you are short sighted, long sighted, or perhaps that you have astigmatism. (If that's not the case, and you still need an eye exam, please find an optician or optical shop near you.

But what do all those numbers on your spectacle 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 spectacle 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 spectacle 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 spectacles, contact lenses and eye medicines, most opticians modernise their prescriptions and use RE (right eye) and LE (left eye) instead of OD and OS.

Other details on your eye prescription

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

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

The term "sphere" means that the correction for shortsightedness or longsightedness 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, either you have no astigmatism, or your astigmatism is so slight that it is not really necessary to correct it with your spectacle lenses.

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 shortsighted astigmatism) or a plus sign (for longsighted astigmatism). Cylinder power always follows sphere power in a spectacle 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 a spectacle 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.

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 dioptres ("p.d." or a superscript triangle when written freehand), prescribed to compensate for eye alignment problems. Only a small percentage of spectacle 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" or 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 dioptres. They are in decimal form and generally are written in quarter-dioptre (0.25 D) increments. Axis values are whole numbers from 1 to 180 and signify only a meridional location, not a power. When prism dioptres are indicated in decimal form, typically only one digit appears after the decimal point (e.g., 0.5).

Additional Information. Your optician also might write specific lens recommendations on your spectacle 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 eye prescription

Confused? Let's use an example to clear things up. (Pun intended.)

Here is a sample spectacle prescription:

RE -2.00 SPH +2.00 add 0.5 p.d. BD

LE -1.00 -0.50 x 180 +2.00 add 0.5 p.d. BU

In this case, the optician has prescribed -2.00 D sphere for the correction of myopia in the right eye (RE, or OD). There is no astigmatism correction for this eye, so no cylinder power or axis is noted. This optician has elected to add "SPH," to confirm the right eye is being prescribed only spherical power. (Some doctors will add "DS" for "dioptres sphere;" others will leave this area blank.)

The left eye (LE, or OS) is being prescribed -1.00 D sphere for myopia plus -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, and this spectacle prescription includes a prismatic correction of 0.5 prism dioptre in each eye. In the right eye, the prism is base down (BD); in the left eye, it's base up (BU).

A glasses prescription is not a contact lens prescription

Spectacle and contact lens prescriptions aren't the same. A spectacle prescription is for the purchase of spectacles 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 a spectacle 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 a spectacle prescription frequently is modified when determining the best contact lens power. One reason is that spectacle lenses are worn some distance (usually about 12 millimeters) from the surface of the eye, whereas contact lenses rest directly on the eye's cornea.

An accurate contact lens prescription can be written only after a contact lens fitting has been performed and the prescribing optician has evaluated your eyes' response to the lenses and to contact lens wear in general.

Your eye prescription is yours to keep

The Optical Consumer Complaints Service (OCCS) clarifies that "an optician is obliged to provide you with the written prescription following the eye examination. You are then able to take the prescription to another practice. 

"By law, the prescription must provide the basic results of the eye examination.

"There are however other measurements or results which are needed to dispense glasses but these will need to be taken by the 'dispensing practice' (practice where you buy the glasses). This can sometimes cause difficulties when purchasing online but the law does not require the examining optometrist to include these details as they may vary depending on the type of frames and lenses selected."

The rule is intended to protect the "portability" of your spectacle prescription, allowing you to use it to buy glasses from the optician of your choice.

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