Ever looked at your eyeglass prescription and thought, “What do all these numbers and letters even mean?” Don’t worry—you’re not alone! Eyeglass prescriptions can look a bit confusing at first, but once you understand the basics, it’s pretty straightforward. Here’s a quick guide to help you make sense of it all, so you know exactly what your eyes need before ordering your next pair from Allegiant.

Your prescription will likely start with OD and OS:

• OD is short for oculus dexter, meaning "right eye."
• OS stands for oculus sinister, which refers to the "left eye."
• Occasionally, you may also see OU (oculus uterque), meaning "both eyes."

2. SPH (Sphere): Correcting Nearsightedness or Farsightedness

The SPH (sphere) number indicates the primary corrective power required for each lens, measured in diopters:

• A negative (-) SPH corrects nearsightedness (myopia), meaning you see objects up close clearly but struggle with distance.
• A positive (+) SPH is for farsightedness (hyperopia), which means you see distant objects well, but close-up tasks might be challenging​

For those with astigmatism, your prescription will include CYL and Axis values:

• CYL corrects astigmatism, an imperfection in the eye's shape that causes blurred vision. A negative CYL value indicates nearsighted astigmatism, while a positive value is for farsighted astigmatism.
• Axis is a number from 1 to 180 degrees, specifying the orientation for the cylindrical correction. For instance, if your CYL is -1.00 with an Axis of 90, this means your astigmatism correction is oriented vertically​

If you see an ADD value on your prescription, it’s the additional magnifying power required for near-vision tasks like reading. This is commonly seen in bifocal or progressive lenses and helps with presbyopia, a common age-related condition affecting near vision. ADD is always a positive number, typically ranging from +0.75 to +3.00 diopters​

Prism correction is not on every prescription. It’s usually prescribed for eye alignment issues, like strabismus or double vision. The Prism value is measured in prism diopters and often paired with a Base direction:

• Base Up (BU) or Base Down (BD) corrects vertical misalignments.
• Base In (BI) or Base Out (BO) corrects horizontal alignment issues​

6. PD (Pupillary Distance): Aligning the Lenses with Your Eyes

PD (pupillary distance) is measured in millimeters and is the space between the centers of your pupils. This measurement ensures the optical centers of your lenses align with your eyes for optimal vision, especially for stronger or progressive prescriptions​

With this guide, you can now understand the main elements in an eyeglass prescription. Knowing what these terms mean is helpful for interpreting the specific needs your eye doctor identified. Remember to always keep an updated prescription, as vision needs can change over time.