If you have ever looked at your prescription for eyeglasses and been completely confused by all of the numbers, abbreviations, and symbols on it-you’re not alone. Prescriptions for the eyes are technical in appearance, but once you know what each value represents, you will understand just how your lenses correct your vision.
What Is an Eyeglass Prescription?
The prescription for eyeglasses is a precise measurement provided by an optometrist or ophthalmologist, which prescribes how lenses shall be made in order to correct your vision. Each number refers to a particular part of your eyesight: clarity, shape of the cornea, eye alignment, and so on.
1. OD and OS – Which Eye Is Which?
These are Latin terms used in eye care:
• OD (Oculus Dexter) – Right Eye
• OS – Oculus Sinister, Left Eye
• OU (Oculus Uterque) – Both Eyes
These markers simply tell the lab which numbers apply to which eye.
2. SPH (Sphere) – Nearsighted or Farsighted?
The SPH value is the main part of your prescription.
What it means:
• Negative (-) SPH → Myopia (clear near vision, blurry distance vision)
• Positive (+) SPH → Farsighted (clear far vision, blurry near vision)
Example:
• -2.00 SPH → moderate myopia
• +1.75 SPH → mild hyperopia
The higher the number, the stronger the lens power required.
3. CYL (Cylinder) – Astigmatism Correction
If you have astigmatism, the CYL number expresses the amount of correction you need.
What it means:
• Astigmatism is a result of an uneven curving of the cornea.
• The value of CYL can be positive or negative.
• If it is blank or has “DS,” then no astigmatism correction is needed.
4. AXIS – Astigmatism’s Direction
AXIS value is present only if the CYL exists.
It shows the orientation of your astigmatism in degrees from 1° to 180°.
Example:
• CYL -1.25 / AXIS 90°
→ The astigmatism correction is vertical.
5. ADD – Reading or Near-Vision Power
ADD stands for “Addition power,” used in:
• Bifocal lenses
• Progressive lenses
• Multifocal lenses
This provides extra magnification for reading and close work, mostly for people over 40 years due to presbyopia.
Typical range:
+0.75 to +3.00
6. PD (Pupillary Distance) – An Important Measurement
PD measures, in millimeters, the distance between the centers of your pupils.
Why it matters:
• Ensures the optical center of your lenses aligns with your eyes.
• Essential for comfort, clarity, and preventing headaches
You may see:
• Single PD – e.g., 62 mm
• Dual PD – for example, 31/31 mm (for each eye)
7. Prism – Eye Alignment Correction
Prism is prescribed when your eyes do not work together properly, resulting in:
• Double vision
• Eye strain
• Image misalignment
The operation shifts light so that less strain will be put on misaligned eyes.
Prism is measured in prism diopters and is typically accompanied by directions such as:
• BI (Base In)
• BO: Base Out
• BU (Base Up)
• BD – Base Down
Why Understanding Your Prescription Matters
Knowing your prescription will help you:
• Select the appropriate type of lens: single vision, bifocal, progressive
• Understand your vision needs
• Order glasses online with confidence
• Spot changes in your eyesight over time
Common Questions About Eyeglass Prescriptions
1. Can I use the same prescription for glasses and contact lenses?
No, for contact lenses a different, more detailed measurement is needed.
2. How long is a prescription valid?
Usually 1–2 years, depending on the local regulations.
3. Can these numbers tell me if my vision is “bad”?
A higher SPH value means stronger correction, but it does not define eye health.