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How to Read Your Eyeglass Prescription

A sample of an eyeglasses prescription
Your prescription has numbers listed under the headings - OS and OD. They are Latin abbreviations: OS (oculus sinister) means the left eye and OD (oculus dextrus) means the right eye. Occasionally, you will see a notation for OU, which means something involving both eyes. However some optometrists would prefer to mention RE – for right eye and LE – for Left eye.

In general, the further away from zero the number on your prescription, the worse your eyesight and the more vision correction (stronger prescription) you need.

A “plus” (+) sign in front of the number means you are farsighted, and a “minus” (-) sign means you are nearsighted.

Farsighted or Hyperopia is a condition when the near objects are blurred, however distant objects appear to be clear, and this condition is corrected using convex lenses. Near sighted or Myopia is a condition when distant objects appear blurred and near objects appear clear and are corrected using concave lenses.

The numbers represent diopters, the unit used to measure the correction, or focusing power, of the lens your eye requires. Diopter is often abbreviated "D."
For example, if your prescription says -1.00, you have one diopter of nearsightedness. This is a fairly mild amount of nearsightedness. If you are -4.25 it means you have 4 and 1/4 diopters of nearsightedness. This is more nearsighted than -1.00, and requires stronger (thicker) lenses. Similarly, +1.00 would be a small amount of farsightedness and +5 would be more.

Prescription for Astigmatism: For people who have astigmatism, there will be three numbers in your prescription. The general form for writing these numbers is S x C x Axis.

The S refers to the 'spherical' portion of the prescription, which is the degree of nearsightedness or farsightedness discussed above.

The C refers to the 'cylinder' or astigmatism, and can be a negative or a positive number. It measures in diopters the degree of astigmatism that you have. The bigger this number, the more astigmatism.

The Axis is a number anywhere between 0 and 180 degrees. It reveals the orientation of the astigmatism. It is not enough to specify how much astigmatism there is; you have to know where the difference in curvature is taking place.
Here are two examples of what prescriptions for eyes with astigmatism could look like:
-2.00 +1.50 x 180
+3.50 +3.00 x 45
The first prescription means that the person has 2 diopters of nearsightedness with 1.5 diopters of astigmatism and an axis of 180 degrees.
The second prescription means that the person has 3.5 diopters of farsightedness, 3 diopters of astigmatism and an axis of 45 degrees.

ADD / ADDITION:
If you are over 40, you may have a number here. This is your reading addition and relates to the amount of additional correction needed to focus at close distances. A measurement will mean you have different prescriptions for distance and reading. Sometimes another add for intermediate prescriptions is also required.
In some cases the near add is written on the prescription in a different format and is already combined to provide the near prescription this is referred as NEAR VISION.

PRISM:
This is the amount of prismatic power, measured in prism diopters prescribed to compensate for eye alignment problems. Only a small percentage of eyeglass 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' (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)
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