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What Do Prescription Numbers Mean in Eye Glasses?

by
author image Beth Richards
Beth Richards, a freelance writer since 2002, writes about health and draws from her 25 years as a licensed dispensing optician. She has authored several books, writes for national magazines including "Country Living" and "Organic Family" and is a health and wellness features writer for several publications. She is earning a Bachelor of Arts in English from the University of Maryland.
What Do Prescription Numbers Mean in Eye Glasses?
The numbers in eyeglass prescriptions contain information for vision correction. Photo Credit ecriture image by Tjall from Fotolia.com

According to Refractive Error Source, it is common to have some degree of imperfect vision because light does not focus correctly on most people's retinas. Most vision issues can be corrected with eyeglasses. Every eyeglass prescription has a set of numbers and abbreviations that are used to identify what is needed to correct a person's vision. The numbers tell whether the prescription is made for nearsighted, farsighted, astigmatism, bifocal or other vision corrections, also called refractive errors.

Features

Eyeglass prescriptions are written in a standardized way to prevent confusion and misinterpretation. The prescription for the right eye--written as OD, which is Latin for Oculus Dexte--is always noted first. OS, or Oculus Sinister, is the left eye. Occasionally a doctor will use the abbreviation RE for Right Eye and LE for left eye, according to The Ohio State University Medical Center. The prescription for each eye includes numbers indicating sphere, cylinder and axis. It sometimes includes other information such as bifocal power or prism correction.

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Sphere

An optical prescription notes whether a person is nearsighted--myopic--or farsighted--hyperopic. This number appears under "Sphere" or "SPH" on a prescription and indicates the power of the spectacle lenses. The power refers to diopter, which is a measure of the refractive strength of a lens or of an eye. The Opticianworks website states that sphere powers in ordinary prescriptions are written with the numbers from +/- 0.25 diopters to approximately +/- 15.00 diopters. Prescriptions above the range of +/- 15.00 diopters may require special lenses.

Nearsightedness is designated by a negative number, for example -1.00, and farsightedness is written as a positive number. The greater the number, the stronger the prescription and power required to see.

Cylinder

When the clear outer layer of the eye is irregularly shaped, the condition is called astigmatism. A perfect cornea is baseball shaped but most corneas tend to be more football-shaped according to the National Eye Institute.



Cylinder numbers in a prescription represent the amount of astigmatism. Cylinder powers in ordinary prescriptions range from +/- 0.25 diopters to approximately +/- 5.00 diopters. Most optometrists write cylinder in the negative form, such as -1.00, and most ophthalmologists write cylinder as a positive number, like +1.00.

The cylinder number also corresponds with the Axis number on a prescription and will be a number between 1 and 180, depending on the curve of the cornea.

It is possible to have astigmatism without nearsighted or farsighted correction. In this case, a person's prescription would have 0.00 written under Sphere and have numbers under cylinder and axis.

Bifocals

Eyeglass prescriptions contain a section after sphere, cylinder and axis called "ADD" which stands for "Addition" or additional power. The numbers in this field indicate the bifocal power of a prescription and are always written as a positive number.



Bifocals usually correct for presbyopia, a condition that begins around the age of 40 to 45, according to optometrist Dr. Ted Montgomery. Loss of elasticity of the crystalline lens causes a reduction in the inability to focus clearly up close. This is different than being farsighted and eventually happens to everyone, even if they are nearsighted or farsighted. The higher the ADD number, the more power required to read.

Prism

Some eyeglass prescriptions include numbers written under the column Prism. This part of the prescription represents a correction for a muscle imbalance in the eye. The Allaboutvision website states that prism is indicated in either metric or fractional English units such as 0.5 or ½, and the direction of the base of prism is indicated by noting it as Base Up or Down (BU or BD) or Base In or Out (BI or BO).

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