You've probably seen abbreviations like OD, OS, SPH, CYL, Axis, and ADD on your prescription before.

Most people hand the prescription to an optician or upload it online without ever knowing what those terms mean.

Here's a quick breakdown:

• OD = right eye  
• OS = left eye  
• SPH (Sphere) = the main correction power for nearsightedness or farsightedness  
• CYL + Axis = measurements used to correct astigmatism  
• ADD = extra magnification for reading in bifocal, trifocal, or progressive lenses

One thing that surprises many people: your glasses prescription and contact lens prescription are not necessarily the same.

Because glasses sit in front of your eyes while contacts sit directly on the eye, the lens powers can differ, especially for stronger prescriptions.

Another detail many people overlook is lens index. If your prescription is relatively strong, higher-index lenses can make your glasses noticeably thinner and lighter.

For a more detailed explanation of every number and abbreviation on an eyeglass prescription:  
  
[https://www.glassesusa.com/blog/how-to-read-your-eyeglasses-prescription](https://www.glassesusa.com/blog/how-to-read-your-eyeglasses-prescription)

Did you already know what all the numbers on your prescription meant, or did you just trust the process like most people do?

How to Read Your Eyeglass Prescription: A Practical Guide