Your prescription is usually represented in a set of numbers and letters. The units in your prescription are used to represent the amount of correction needed in order to normalize vision for distance. These units are called ‘diopters’. The more severe your disorder (e.g. nearsighted or farsighted) is, the higher your prescription is in diopters.

Your prescription is usually written in three numbers. For example:

  • OD -4.25 -1.75 X 180
  • OS -5.50 -1.25 X 175

Here is the way to decipher your prescription, using the example from above:

  • OD stands for right eye, and is the abbreviation for the Latin Ocular Dexter. OS is for the left eye. It is derived from the Latin Ocular Sinister.
  • The First Number (-4.25 and -5.50 in this example) is the degree of spherical nearsightedness or farsightedness. The sign identifies whether you are nearsighted ( – sign) or farsighted (+ sign).
  • The Second Number  (-1.75 and -1.25) is the degree of astigmatism. The number can be written either with a + sign or a – sign.
  • The last and Third Number (180 and 175) is the axis, or the direction of your astigmatism. An axis of 180 degrees, for example, means the astigmatism is horizontal.
  • Therefore, this prescription means that the patient is moderately nearsighted, with a moderate degree of astigmatism in a horizontal direction.
  • Some people only have one number written for each eye. This is when there is no astigmatism.

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