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Breaking Down the Terminology of a Lens Prescription

What does OD and OS Mean? No clue? You are not alone! Understanding how to read your eyeglass prescription is still a fine art unknown to many! The first step is knowing what “OD” and “OS” mean. They are short 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 OculusUterque, which means “both eyes.”
 

The values under the column “Sphere (SPH)” indicate the amount of lens power, measured in diopters (D), prescribed to correct your 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. To understand this better, imagine cutting a slice off a glass ball. The slice would be shaped like a dome, with the same curve in all meridians of the dome. The original shape from which you cut the slice was a ball or “sphere”, which gives this component of a lens prescription its name.
 

The value in the “Cylinder (CYL)” column indicates the amount of lens power for astigmatism. If nothing appears in this column, you have no astigmatism. Imagine cutting a slice from a glass rod, along its length. The slice would resemble a long “domed” glass ruler. The original shape from which you cut the slice was a rod or “cylinder”, which gives this component of a lens prescription its name. The number in the cylinder column is the “power” of the cylinder. It 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.
 

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. The cylinder “axis” of a prescription specifies in degrees how the cylinder component should be oriented in front of the eye.
 

The value in the “Multifocal Add” column is the magnifying power “added” to the bottom part of multifocal lenses to aid near vision. 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. Adds are normally prescribed for lens wearers over age 40 but (in special cases) they may be prescribed for younger wearers as well.
 

An Example of an Eyeglass Prescription

OD -2.00 SPH +2.00 add OS -1.00 -0.50 x 180 +2.00 add
 

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, combined with -0.50 D cylinder for the correction of astigmatism. The cylinder power has its axis at the 180 meridian. Both eyes are being prescribed an “add power” of +2.00 D to aid near vision.
 

An Eyeglass Prescription Is Not a Contact Lens Prescription

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. In stronger prescriptions, the power of the prescription can even be different for contacts than for glasses.
 

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.