Have you ever really know what your optical prescription (Rx) means? Many people have been wearing eyeglasses since early in life and have little to no idea. How do you know which is for the right eye versus the left eye? Why does your prescription have one set of numbers, but your friend's has three? The answers to these questions and more are easily explained.
In the example below, let's start with the terms "OD" and "OS". These are abbreviations taken from the latin words oculus dextrus, meaning "right eye", and oculus sinister, meaning "left eye". Optometrists and ophthalmologists have used these terms within the optical community for years. You may see another term, "OU", which means oculus utro and refers to an Rx or condition involving both eyes.
Now on to the numbers themselves. A good rule of thumb when reading a prescription is this: the further from zero each number is, the worse your eyesight. This could mean a number of things, from needing higher quality eyeglass lenses to requiring toric contact lenses for an astigmatism.
The first number represents the diopter, which is the unit measuring the correction needed for the eye to properly focus. A plus sign means you're farsighted, or hyperopic. A minus sign means you are nearsighted, or myopic. If there are three numbers in your prescription you have an astigmatism, or an irregularly shaped cornea requiring further vision correction. The second and third numbers in the line refer to the "cylinder", or astigmatism itself, and the "axis", the orientation of the astigmatism on the cornea.
The sample prescription above has four numbers total, because this is for a bifocal or progressive lens prescription. The final number corrects for presbyopia, a condition that typically develops in the early 40s requiring an Rx for reading up close.