🧾 How to Read Eye Test Prescription: A Simple Guide from an Optometrist
When you walk out of an eye clinic with your prescription in hand, it might look like a secret code: full of pluses, minuses, and abbreviations like OD, OS, SPH, CYL, and AXIS. If you've ever stared at that piece of paper thinking, "How on earth am I supposed to understand this?" — you’re not alone.
As an optometrist, I’ve explained prescriptions to hundreds of patients — so today, let me break it down like I would explain to a friend or family member. Simple, easy, and real.
🔍 Quick Overview: What is an Eye Test Prescription?
An eye test prescription tells you the type and amount of vision correction you need. It’s like a map for glasses or contact lenses, showing how to adjust your lenses so your vision becomes clear.
Every part of that prescription is based on measurements taken during your eye exam — no guesswork involved. But to make sense of it, you need to know what each part means.
👁️ First: What Do OD and OS Mean?
These are Latin abbreviations we still use in the eye world:
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OD = Oculus Dexter = Right Eye
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OS = Oculus Sinister = Left Eye
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OU = Oculus Uterque = Both Eyes
So, when you see your prescription table, OD is the right eye row, OS is the left.
🧠 Key Terms in Your Eye Prescription Explained
| Term | Full Form | Meaning |
|---|---|---|
| SPH | Sphere | The lens power you need for nearsightedness (-) or farsightedness (+) |
| CYL | Cylinder | The amount of astigmatism correction (if any) |
| AXIS | Axis | Direction (in degrees) where the astigmatism lies |
| ADD | Addition | Extra power for reading (used in bifocals/progressives) |
| PD | Pupillary Distance | Distance between your pupils (used to center your lenses) |
| Prism/Base | Prism Correction | Used in rare cases for eye alignment issues |
👓 Let's Decode a Real Example
Here’s what a sample prescription might look like:
| Eye | SPH | CYL | AXIS | ADD |
|---|---|---|---|---|
| OD | -2.00 | -1.00 | 180 | +1.50 |
| OS | -1.75 | -0.50 | 170 | +1.50 |
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OD (Right Eye): You're nearsighted with -2.00 power. You also have astigmatism (-1.00) at 180 degrees.
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OS (Left Eye): Slightly less nearsighted, with -1.75, and milder astigmatism.
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ADD: +1.50 means you need additional power for reading or near tasks (usually for people 40+).
✅ How to Know If You're Nearsighted or Farsighted
It’s all in the SPH (Sphere):
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Negative (-) = Nearsighted (you see near objects clearly, distant ones are blurry)
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Positive (+) = Farsighted (you see far objects clearly, near ones are blurry)
The bigger the number, the stronger the prescription.
🔄 Understanding Astigmatism: CYL and AXIS
Astigmatism isn’t a disease. It just means your eye isn’t perfectly round — it’s more like a football than a basketball.
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CYL (Cylinder) tells how much correction is needed.
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AXIS (0–180 degrees) shows where the irregular curve is on your eye.
These two always appear together. If there’s no astigmatism, the CYL and AXIS will usually be blank or marked “DS” (diopters sphere only).
➕ What is ADD in a Prescription?
This part is mostly for people over 40 who need help reading or using a phone.
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ADD shows extra power added to your prescription for near tasks.
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It’s used in bifocal or progressive lenses.
Think of it like this: SPH handles your general vision, ADD boosts your reading vision.
👁️🗨️ PRISM – Rare but Important
If your eyes don’t align properly (a condition called strabismus), your optometrist might add prism correction.
This helps guide the image to the right spot on the retina, so both eyes work together.
You’ll see this as:
| Prism | Base Direction |
|---|---|
| 1.5 Δ | Base Out |
📏 What is PD (Pupillary Distance)?
This is the distance between your pupils, measured in millimeters. It ensures that your lenses sit right in front of your pupils.
Without accurate PD:
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Glasses might feel off or cause strain
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Lenses may not work properly, especially in higher powers
Your optometrist or optical store usually measures it, or you can use an online PD tool (though it's better done by a pro).
🧠 Fun Fact: Glasses and Contact Lens Prescriptions Aren’t the Same
Yes, they’re different!
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Glasses prescription is for lenses that sit ~12mm from your eye.
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Contact lens prescription includes:
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Base curve
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Diameter
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Brand
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And sometimes, slightly different SPH/CYL values.
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If you wear contacts, don’t try to use your glasses Rx — you need a contact lens fitting.
🛠️ Why It’s Important to Understand Your Eye Prescription
Understanding your prescription helps you:
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Ask better questions at your eye exam
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Order online glasses more confidently
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Track changes in your vision year-to-year
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Spot mistakes in online orders (yes, they happen!)
And honestly, it just feels empowering to know what those numbers mean.
🤔 FAQs: How to Read Eye Test Prescription
❓ Can I use my eye prescription to order glasses online?
Yes, as long as you have PD and the latest prescription (within 1–2 years).
❓ Can I convert glasses Rx to contacts?
No — contact lenses need a separate fitting due to different measurements and materials.
❓ What if there's no CYL or AXIS on my Rx?
It means you don’t have astigmatism correction. Only SPH is needed.
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✍️ Final Words from Me
You don’t need to be an optometrist to understand your eye prescription. With a little explanation, it all becomes simple and clear. So next time you get your eye test, take a few minutes to look at your numbers and ask questions.
And remember — no online blog or tool can replace an in-person eye exam. Always check with your optometrist if you’re unsure.
Stay curious, stay clear. 👓
