An eyeglass prescription is written in a standardized format however sometimes they can be hard to understand. Here are what the numbers mean:
LEFT AND RIGHT EYE
The letters OD and OS in front of a prescription let us know which eye each string of numbers is for.
OD (Oculus Dexter) = right eye
OS (Oculus Sinister) = left eye
OU (Oculus Uterque, Oculus Unitas or Oculus Uniter) = both eyes.
SPH - Sphere or Spherical
The first number in the prescription is the Sphere (or Spherical) and it is the spherical refractive error (farsightedness or nearsightedness). When there is a minus sign in front of the SPH number, the patient is nearsighted (meaning they would need glasses to see things that were far away like the TV). A plus sign would indicate someone who was farsighted (meaning they had trouble reading a menu or their watch).
CYL – Cylinder
The second number in the prescription is the Cylinder and it is used to indicate a correction for astigmatism. (An eye that is no longer round but more like an egg or football shape). If there is no astigmatism, you may see a zero or the letters DS or SPH after the first number to let the optician know that the doctor didn’t just forget to write in the astigmatism.
AXIS
The last number in a basic prescription is the Axis or direction of the astigmatism. Astigmatism, a football-shaped eye, can be measured in any direction around the clock. The Axis numbers indicate the orientation of the football shape.
ADD
There may be additional numbers in a glasses prescription. The ADD numbers denote the amount of near-reading strength needed in a bifocal or progressive lens. They usually go from +0.75 to +3.00, depending on age and visual need.
PRISM
A Prism correction is used to treat muscular imbalance or other conditions and the numbers are usually left blank on the prescription and not many people need this correction. Sometimes, if the basic prescription is followed by a small number with a superscript (1^) it indicates prism correction. There may be more than one set of prism numbers for each eye.
The following are a few examples of sample prescriptions:
Note that each of the following examples is for the same eyeglasses prescription. Also note that the ADD, which is for the reading portion of a bifocal or progressive lens is not in the last example because there is a separate NV or Near Vision portion for this prescription.
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