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Your Prescription
When you look at your prescription for
glasses, you will see numbers listed under the headings of OS and
OD. They are Latin abbreviations; OS means the left eye and OD
means the right eye. Occasionally you will see a notation for OU,
which means something involving both eyes.
In general, the further away from zero
the number on your prescription, the worse your eyesight and the
more correction you need. A plus sign in front of the number means
you are farsighted (hyperopic) and a minus sign means you are
nearsighted (myopic). 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, 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.
If your eye has astigmatism (two
different corneal curvatures), there will be three numbers in your
prescription. The general form is S x C x Axis.
The S refers to the "spherical"
portion of the prescription – the nearsighted or farsighted
numbers discussed above.
The C refers to the "cylinder" or
astigmatism, and can be a negative or a positive number. The
bigger this number, the more astigmatism you have.
The Axis is a number anywhere
between 0 and 180, and it refers to degrees. It reveals where the
difference in corneal curvature or the orientation of the
astigmatism occurs. It is not enough to specify how much
astigmatism there is, you have to know where the difference in
curvature is taking place.
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