You have just had your vision exam and your optometrist or ophthalmologist has given you a prescription or prescription for glasses. He probably told you that he was nearsighted or farsighted, or perhaps had astigmatism.

But what do all those numbers mean on the eyeglass prescription? What do the abbreviations OD, OI, SPH and CYL mean?

This article will help you decipher all the elements of the prescription and speak more knowledgeably with your eye care professional when shopping for glasses.


What do OD and OI mean?

The first step in understanding the prescription of glasses is to know what OD and OI mean. They are the abbreviations for right eye and left eye, respectively.

The eyeglass prescription may also have a column labeled AO. This is shorthand for the term both eyes.

In eyeglass prescription, right eye (RE) information comes before left eye (LE) information. Eye doctors write prescriptions this way because when they are in front of the patient the first thing they see is the right eye to the left and then the patient's left eye to the right of them.

More terms in the prescription of glasses
Eyeglass prescription also has other terms and abbreviations, including:

Sphere (Sphere - SPH)

Indicates the amount of lens power, measured in diopters (D), and is prescribed to correct nearsightedness or farsightedness. If the number under this heading has a minus sign (–), you are nearsighted; if the number has a plus sign (+) or is preceded by neither a plus sign (+) nor a minus sign (–), then 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 (Cylinder - CYL)

Indicates the amount of lens power for astigmatism correction. If nothing appears in this column, you do not have astigmatism; or, that the astigmatism is so slight that it is not really necessary to correct it with the lenses of your glasses.

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

Meridianos del ojo Snellenvision

The meridians of the eye are determined by superimposing a protractor on the front surface of the eye. The 90 degree meridian corresponds to the vertical meridian of the eye and the 180 degree meridian corresponds to the horizontal meridian.


Describes the lens meridian that does not contain cylinder power to correct astigmatism. The axis is defined by 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 the eyeglass prescription includes the cylinder power, it must also include the axis value, which follows the right side of the cylinder power and is preceded by an "x" when written by hand.

The axis is the meridian of the lens that is 90 degrees from the meridian containing the cylinder power.


Addition (ADD)

This is the magnifying power that is added to the bottom of multifocal lenses to correct presbyopia. The number that appears in this section of the prescription is always positive power (+), even if it is not preceded by a plus sign (+). Generally, it ranges from +0.75 D to +3.00 D and will be the same power for both eyes.


This is the amount of prismatic power. It is measured in prismatic diopters and if the prescription is written by hand, a superscript triangle is drawn. It is prescribed to compensate for eye alignment problems. Only a small percentage of eyeglass prescriptions include prisms.

Sphere power, cylinder power, and add power are always expressed in diopters. They are in decimal format and are usually written in quarter diopter increments (0.25 D). Axis values are integers from 1 to 180 and indicate southern location only and not power.

Additional Information

In order to give you the most comfortable vision correction possible, your eye doctor may also recommend specific lenses in your eyeglass prescription, such as anti-reflective coatings, photochromic lenses, or progressive lenses.