When you have your eye exam and the doctor gives you a copy of your eyeglass prescription, do you know how to read it? What does OD, OS, and OU mean?
An eyeglass prescription is written in a standardized format with standardized notation so it can be interpreted worldwide. Let’s look at one and break it down:
-2.00 -1.00 x 180. The first number (-2.00) tells us the spherical refractive error (farsightedness or nearsightedness). In this case, because there is a minus sign in front of the 2.00, this patient is nearsighted. A plus sign would indicate farsightedness.
The second number (-1.00) is the astigmatism. If there is no astigmatism, we generally write the letters DS or SPH after the first number to let the optician know that we didn’t just forget to write in the astigmatism.
The final number (180) is the direction (AXIS) of the astigmatism. Astigmatism, a football-shaped eye, can be measured in any direction around the clock. We use the numbers from 90 to 180 to indicate the orientation of the football shape.
There may be additional numbers in a glasses prescription. For instance, 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.
Lastly, there can also be numbers denoting the amount of near reading strength needed (bifocal or progressive). They usually go from +0.75 to +3.00, depending on age and visual need.
The letters OD and OS in front of a prescription let us know which eye each string of numbers is for. OD (Oculus Dexter) stands for right eye and OS (Oculus Sinister) for left eye, while OU (Oculus Uterque, Oculus Unitas or Oculus Uniter) means both eyes.

Sphere: This number measurement reflects the extent of the nearsightedness or farsightedness.
Cylinder: This number measurement refers to the amount of astigmatism (an irregularly shaped cornea which causes blurring) in the eye.
Axis:This number measurement describes the astigmatism in degrees from the horizontal (most left and right eyes have the same axis in astigmatism) axis.