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Eyeglass Prescriptions

How to Read Your Eyeglass Prescriptions

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Updated June 02, 2009

Eyeglass Prescriptions

Eyeglass prescription

© 2009 Troy Bedinghaus licensed to About.com, Inc
If you've ever looked closely at your eyeglass prescriptions, you've probably wondered how to make sense of all those numbers and symbols. In order to be interpreted worldwide, eyeglass prescriptions are written in a standardized format with standardized notation. Here's a look at a sample eyeglass prescription and a walk-through on how to read it:

Example:

OD: -2.00 – 0.50 x 180

OS: +1.00 DS

ADD: +1.75 OU

A Lesson in Latin

One must first take a little lesson in Latin to make sense of an eyeglass prescription. Latin abbreviations are often used in health care to write medical prescriptions and eyeglass prescriptions. These abbreviations are becoming less and less common because state and federal rules and regulations are starting to reduce dependence on them.

The letters OD stand for "oculus dexter" and indicate the right eye. The letters OS stand for "oculus sinister" and indicate the left eye. The letters OU stand for "oculi uterque" and refer to both eyes.

Numbers, Numbers, Numbers

Eyeglass prescriptions contain numbers -- lots and lots of numbers. In our example above, the first number to the right of OD is -2.00. This is considered the "sphere" part of the prescription. The sphere number indicates nearsightedness or farsightedness. Generally, a minus sign (-) indicates a negative-powered lens that is used to correct nearsightedness. A positive sign (+) indicates a positive-powered lens used to correct farsightedness.

The next number in the sample eyeglass prescription is -0.50. This number represents the "cylinder" part of the prescription. Another word for cylinder is astigmatism and represents the difference in curvature and power between two points on the eye, separated by 90 degrees. The next number is x 180, read as "axis 180." This number indicates the angle in degrees from 0 to 180, representing the location of the most positive meridian in an eye that has astigmatism when written in minus cylinder form, as in the example above.

For the left eye, you will notice that the "sphere" number is plus one (+1.00) DS. The letters DS refer to "diopters sphere" and indicate that the left eye’s correction is totally spherical in nature with no astigmatism. In other words, the right eye’s cornea probably has a slightly oblong shape, and the left cornea’s shape is very close to being perfectly round. It is common to write SPHERE or DS as a place holder in the astigmatism number’s place to make sure that the reader of the prescription knows that the doctor did not accidently forget to record the cylinder or astigmatism correction.

Finally, the ADD number of +1.75 represents the power that needs to be "added" to the distance prescription to give the patient clear vision at a close range for reading and near point activities. This number is usually absent in younger people's eyeglass prescriptions. While some young people can develop near focusing problems and need an "add" power, it usually indicates that a patient needs a bifocal power as he starts to loose his near focusing ability (a problem that commonly begins to develop as we reach our forties.)

Many people mistakenly believe that +1.75 is the power needed when purchasing over-the-counter reading glasses. However, you have to do a little algebra to figure the total power needed for single vision reading glasses. In the example above, it is -2.00 +1.75 = -0.25. So the prescription for reading glasses for the right eye would be -0.25 – 0.50 x 180. For the left eye, the calculation is +1.75 +1.00 = +2.75 D.S.

Note that most people have prescriptions unlike the example shown above and usually have numbers similar in power for both eyes. This example was chosen to show the difference between nearsighted and farsighted prescriptions and how these numbers affect calculations often made by opticians, optometrists and ophthalmologists.

More Abbreviations

The following are a few other abbreviations you may encounter on your eyeglass prescription:
  • SVD - Single Vision Distance, or glasses for distance only

  • SVN - Single Vision Near, or glasses for reading only

  • Sphere - Spherical power has the same power in all meridians

  • Cylinder - A cylinder power corrects astigmatism and represents the difference in the greatest power of the eye and weakest power of the eye, usually separated by 90 degrees.

  • Axis - indicates the angle (in degrees) between the two meridians of an astigmatic eye

  • PD - (pupillary distance, or distance between the centers of the two pupils between the eyes) This measurement is essential to designing glasses that comfortable to wear and optically perfect.

  • Prism - Prism is not commonly prescribed. It is often prescribed to displace the image in a certain direction for patients with crossed-eye (strabismus) or other eye muscle or focusing disorders.

As you can see, reading your eyeglass prescription can be confusing. However, with a little study and practice, you'll be able to read your eyeglass prescription like the pros if you're ever in need.

Source:

Eskridge, J. Boyd, John Amos and Jimmy D. Bartlett. "Clinical Procedures In Optometry." Copyright 1991 by J.B. Lippincott Company, Chapter 18 - "Monocular Subjective Refraction" by Polasky Michael, pp 174-188.

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