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The four main vision focusing disorders of the eye are:
myopia (nearsightedness) astigmatism (ovalness of the eye) hyperopia (farsightedness) and presbyopia (inability to change the focus from far to near)
The units used to represent the amount of correction needed in order to normalize vision for distance are called 'diopters'. The more nearsighted or farsighted you are, 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
+2.25 Add OU
represents a typical prescription.
Here is the way to decipher your prescription:
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 1st 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 3rd 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.
The "add" at the bottom of the prescription is for the reading part of a bifocal glass. It might be unusual for anyone under the age of 40 to need this.
Some people only have one number written for each eye. This is when there is no astigmatism.
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