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  • After an eye doctor has measured the degree of your nearsightedness, farsightedness and/or astigmatism, you may be given a prescription to correct your vision. In general, the farther away a number in your prescription is from 0, the more vision correction you will require.

    OS and OD

    The abbreviation OS stands for the Latin "oculus sinister," referring to the left eye. OD, or "oculus dextrus," refers to the right eye. OU is sometimes used to indicate something that involves both eyes.

    Plus and Minus

    According to Web MD, a plus sign (+) in front of the number indicates that you are farsighted, and a minus sign (-) in front of the number indicates that you are nearsighted.

    Diopters

    The unit of measure for vision correction is called a diopter. As an example, a prescription of -1.00 means that you have 1 diopter of nearsightedness. A prescription of -4.25 means you have 4 and 1/4 diopters of nearsightedness, requiring a stronger (and thus thicker) lens to correct your vision.

    Astigmatism

    Three numbers are used to indicate astigmatism in a lens prescription. The first number refers to the degree of farsightedness or nearsightedness. The second number, which may be either negative or positive, refers to the degree of astigmatism. The third number, between 0 and 180 degrees, indicates the axis or orientation of astigmatism.

    Prescription Example

    An example of a prescription for a person with astigmatism might be -3.75 +1.00 x 50, meaning that he has 3 and 3/4 diopters of nearsightedness, 1 diopter of astigmatism and an axis of 50 degrees.

    Source:

    WebMD: How to Read Your Eyeglass Prescription

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