Many patients want to understand what the numbers signify on a prescription for glasses or contact lenses. Unlike an eye test, an optical prescription is not a direct measurement of the ability to see. It does not compare eyesight with that of someone who has perfect vision, the way an eye chart does. Instead, the prescription is a specification of the power required to correct vision to normal.
Refractive errors occur when light rays entering the eye cannot focus precisely on the middle of the retina. When this happens the retina is incapable of relaying a clear message to the brain and you can’t see clearly. Corrective lenses bend the light rays to focus more directly onto the retina. The amount of correction necessary is written as a prescription.
The first number indicates myopia or hyperopia:
Mild to moderate myopia = in the range of -1 to -6 diopters.
Mild to moderate hyperopia = within the range of +0.75 to +4.00 diopters.
A second number indicates both the presence of astigmatism and the degree of it. Astigmatism may have either a minus sign or a plus sign. Most everyone has some degree of astigmatism.
A third number, called axis, indicates the direction of astigmatism
An example might look like:
It would mean:
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