Many people probably know that 20/20 vision is a good thing. Beyond this fact, however, eyeglass prescriptions can be a little bit complicated to understand. All those letters and numbers can feel like you need special coding skills to decipher them, but it doesn’t have to be that way. So, what exactly do the numbers in your prescription mean? Allow us to clear this up. 

Prescriptions are made explicitly for either individual eyes or both eyes. In the prescription, eyes are referred to as “OD,” “OS,” or “OU.” “OS” means Oculus Sinister or the Left Eye. “OD” is Oculus Dexter or the Right Eye. Occasionally, “OU” is used to refer to both eyes. Optometrists generally indicate a unique prescription for each eye because it is common for people to have different levels of clarity for each. 

Diopter or “D” is the measurement for optics. It is represented by numbers, and these are the ones usually seen in any prescription. These numbers signify focal lengths in meters, as they indicate the focusing power required for the lens of your eye to see clearly. The general rule of thumb is that the further the number is from zero, the worse your eyesight condition is and the more correction will be needed. The higher the number, the more powerful the lens required to help you achieve 20/20 vision. 

There are also standard abbreviations and terms that you will find in the prescription, such as:

Sphere (SPH)

The sphere shows the strength of the lens required to correct your eye focus. A negative sphere (-) refers to short-sighted prescriptions (Myopia). This means that you can see things close by, but objects at a distance are blurry. A positive sphere (+) refers to far-sighted prescriptions (Hypermetropia), meaning that you can see things at a distance, but the objects nearby are unclear. 

Let’s break it down further with examples to help you better understand the numbers in your prescription. If your prescription says -1.00 in the OS and -4.00 in the OD, it means that your left eye has one diopter of nearsightedness, and your right eye has 4 diopters of nearsightedness, which will require stronger lenses compared to the left. Similarly, if it reads +1.00, this signifies 1 diopter of farsightedness, a small amount compared to +4.00.

Cylinder (CYL) and Axis

If there is a diagnosis of astigmatism, you will see more numbers in the prescription. Astigmatism simply means that the eye has difficulty focusing at certain angles. If this condition is present, aside from “SPH,” “CYL” and “Axis” are also noted in the prescription. 

A cylinder or “CYL” is the lens power needed to correct for astigmatism. Axis is the positioning of astigmatism and this ranges from 1 to 180. 

Here is an example of a prescription for eyes with the condition of astigmatism: 

OS: -1.00 -1.00 x 180 

OD: +1.00 -3.00 x 50 

This prescription means that the person’s left eye has 1 diopter of nearsightedness with 1 diopters of astigmatism at an axis of 180 degrees. The person’s right eye has 1 diopter of farsightedness with 3 diopters of astigmatism at an axis of 50 degrees. It’s important to remember that these guidelines apply only to eyeglass prescriptions. Contact lenses require a different power due to their placement, given that they are put directly onto the eyes themselves. 

If you have questions or concerns regarding your prescription, please reach out and contact us. We’ll do our best to help you and your family see the world as clearly as possible.