How To Read Your Prescription Like A Pro
How to Read Your Glasses Prescription Like a Pro ๐
You've had your eyes tested, the optometrist hands you a slip of paper with a grid of numbers and abbreviations โ and it looks like a foreign language. What does SPH mean? What's the difference between OD and OS? And why does your prescription have a minus sign in front of some numbers?
This guide breaks down every value on your prescription in plain language, so you can order glasses online with complete confidence โ no guesswork, no second-guessing.
๐ A Sample Prescription
| Eye | SPH | CYL | AXIS | ADD | PD |
|---|---|---|---|---|---|
| OD (Right) | -1.50 | -0.75 | 180 | +1.50 | 63 mm |
| OS (Left) | -1.25 | -0.50 | 175 | +1.50 |
๐๏ธ The Two Eyes: OD and OS
Almost every prescription starts with two rows โ one for each eye. The labels used are Latin abbreviations that optometrists have used for centuries:
- OD (Oculus Dexter) โ your right eye
- OS (Oculus Sinister) โ your left eye
- OU (Oculus Uterque) โ both eyes together (less common in glasses prescriptions)
Right eye is always listed first, left eye second. When entering your prescription online, make sure you're filling in the correct values for the correct eye.
๐ What Each Term Means
The most important number on your prescription. It tells you the overall strength of correction your eyes need, measured in dioptres.
- A negative SPH (e.g. -1.50) means you are short-sighted โ you struggle to see things far away clearly
- A positive SPH (e.g. +2.00) means you are long-sighted โ you struggle to see things up close clearly
- 0.00 or PL (Plano) means no spherical correction is needed for that eye
SPH values increase in steps of 0.25. A prescription of -0.50 is mild; -6.00 is strong.
The CYL value appears when you have astigmatism โ a condition where the eye is slightly oval-shaped rather than perfectly round, causing blurred or distorted vision at any distance.
Not everyone has astigmatism. If your CYL column is blank or shows 0.00, no correction is needed. CYL always appears together with an AXIS value โ one cannot exist without the other.
The AXIS tells your lens maker the orientation of your astigmatism correction. It's measured in degrees from 1 to 180 โ it has no unit (no mm or dioptres), it's purely directional.
Enter this exactly as written โ even a small error in the AXIS can noticeably affect your vision.
The ADD value appears on prescriptions for people who need different strengths for distance and near vision โ typically people over 40 developing presbyopia (age-related difficulty focusing up close).
ADD is always a positive number, usually between +0.75 and +3.50. If your prescription includes ADD, you'll need bifocal or multifocal lenses. If ADD is blank, single vision lenses are correct for you.
The distance in millimetres between the centres of your pupils. It's not a vision correction measurement โ it's a positioning measurement that ensures your lenses are centred correctly in your frame.
An incorrect PD can cause eye strain and discomfort even when the rest of the prescription is perfect. See our PD measurement guide if it's not included on your prescription.
๐งฉ A Sample Prescription โ Decoded
OD (Right Eye): SPH -1.50 / CYL -0.75 / AXIS 180 / ADD +1.50
OS (Left Eye): SPH -1.25 / CYL -0.50 / AXIS 175 / ADD +1.50
PD: 63mm
Reading this: both eyes are short-sighted (negative SPH), both have mild astigmatism (CYL and AXIS present), and both have an ADD value โ meaning this person needs multifocal or bifocal lenses. Their PD is 63mm.
โ Common Questions About Prescriptions
In South Africa, glasses prescriptions are generally valid for two years from the date of issue. If your prescription is older than two years, we recommend getting a fresh eye test before ordering.
No โ contact lens and glasses prescriptions are different. Contact lenses sit directly on your eye, while glasses sit approximately 12mm in front. This distance affects how the prescription needs to be calculated, particularly for stronger prescriptions. Always use a glasses-specific prescription when ordering frames.
That's perfectly normal. Many people have astigmatism in one eye but not the other. Simply leave the CYL and AXIS fields blank for the eye with no astigmatism when ordering online.
Higher prescriptions generally result in thicker lenses. If your SPH is above +/-4.00, you may want to consider high-index lenses which are thinner and lighter than standard lenses. Message us on WhatsApp if you're unsure which lens index is right for your prescription.
๐ Ready to Order?
Once you've found a frame you love, we'll guide you step-by-step to enter your prescription during checkout. Our lens flow system is built by qualified optometrists โ fast, secure, and 100% optometrist-approved. No guesswork needed!
Still not sure? Just upload your prescription during checkout โ we'll do the rest. Your perfect pair awaits. โจ
