Your Prescription

What do all the numbers on my prescription mean?

OD and OS

When you look at your prescription for prescription glasses, you will see numbers listed under the headings of 'OS' and 'OD'. The term 'OS' (oculus sinister) means the left eye and 'OD' (oculus dextrus) means the right eye.

Most prescriptions will simply have R and L - meaning Right Eye and Left Eye.

The further away from zero the number on your prescription, the worse your eyesight and the more vision correction, a stronger prescription you will need.

A 'plus' (+) sign in front of the number means you are far sighted. A 'minus' (-) sign means you are short sighted. These numbers represent diopters. A Diopter is the unit used to measure the correction, or focusing power, of the lens your eye requires. The term Diopter is often abbreviated "D."

Sphere, Cylinder and Axis

Some people have astigmatism. This is caused by a deviation from the normal spherical curvature which results in distorted images. In this case there will be three numbers in your prescription. The general form for writing these numbers is seen as S x C x Axis

The 'S' refers to the "spherical" portion of the prescription, which is the degree of near-sightedness or far-sightedness.

The 'C' refers to the "cylinder",or astigmatism, and is a negative number. It measures in diopters the degree of astigmatism that you have. The bigger this number the more astigmatism you have. Astigmatism most often is caused by a cornea that is shaped more like a rugby ball than a soccer ball.

The 'Axis' is a number anywhere between 0 and 180 degrees. It reveals the orientation of the astigmatism. It is not enough to specify how much astigmatism there is. You have to know where the difference in curvature is taking place.


Here are two examples of what prescriptions for eyes with astigmatism could look like:

Prescription Example #1:

-1.50 +1.50 x 135

This means that the person has 1.5 diopters of near sightedness with 1.5 diopters of astigmatism and an axis of 135 degrees.

Prescription Example #2:

+3.00 +2.50 x 45

This means that the person has 3 diopters of far sightedness, 2.5 diopters of astigmatism and an axis of 45 degrees.