How to Read Your Eyeglass Prescription (The Only Guide You'll Ever Need)
Most people receive their eyeglass prescription, glance at the numbers, and think:
"Okay… but what does all this actually mean?"
Good news. Your eyeglass prescription is easier to understand than it looks. Those numbers are simply measurements describing how light needs to be redirected so your eyes can see clearly. But here's the part most people don't realize: an eyeglass prescription does not tell the full story.
It tells us the power your lenses need, but not how to fit, measure, adjust, centre, or structure them into a pair of glasses that actually feel good to wear. That's where your optician comes in.
In this guide, we'll walk you through what each section of your prescription means, why PD alone is not enough, and why professional measurements are essential for comfort, accuracy, and long-lasting visual clarity.
What Exactly Is an Eyeglass Prescription?
An eyeglass prescription is a medical document written by your optometrist or ophthalmologist. It outlines:
The type of vision correction you need
The lens power required
Any additional magnification for reading
How your eyes differ from one another
Whether you need correction for astigmatism
Your prescription is determined during a comprehensive eye exam, where your eye doctor assesses not only your vision, but also the health of your eyes and how your visual system works together. It reflects how your eyes focus light, not just on a chart, but in real life.
How to Read the Main Parts of Your Eyeglass Prescription
Understanding your prescription starts with a few key abbreviations.
OD and OS: Right vs. Left Eye
OD = oculus dexter → right eye
OS = oculus sinister → left eye
Your eyes rarely have identical needs, so each side gets its own values.
SPH (Sphere): Nearsighted or Farsighted?
SPH tells you the main lens power your eyes need.
− (minus) → myopia (nearsightedness)
You see well up close but blurry at a distance.
+ (plus) → hyperopia (farsightedness)
You see better far away than up close.
Example:
SPH −2.00 means you need a −2.00 diopter lens to see clearly far away.
CYL and Axis: Your Astigmatism Correction
Astigmatism simply means your eye isn't perfectly round.
CYL (Cylinder): the amount of astigmatism
Axis: the angle (0° to 180° - like a protractor) where the correction is placed
Example:
CYL −1.25 Axis 90°
→ You need 1.25 diopters of correction at the 90-degree meridian.
Astigmatism is extremely common, and even small changes can dramatically affect clarity and comfort.
ADD: Extra Power for Reading
If you see ADD, NV ADD, or Near Add, it means you need additional magnifying power for near tasks. Reading add lenses are very common for people with presbyopia, or difficulty focusing, or those who spend extended time on near-point tasks and those who are feeling visually fatigued. This is used in:
Progressive lenses
Bifocals
Reading glasses
Anti-fatigue lenses
Example:
ADD +2.00 means you need +2.00 diopters more for comfortable near vision.
Prism: Helps with Eye Alignment
If you see a value listed under Prism, it means your lenses are correcting an eye alignment issue, often to help with double vision or eyes that do not work perfectly together. Prism is measured in prism diopters and is usually written with a direction such as BU (base up), BD (base down), BI (base in), or BO (base out).
Prism does not change how strong your prescription is for clarity. Instead, it gently shifts the image so that both eyes can aim at the same point more comfortably. This part of the prescription is highly customized and must be measured and verified carefully by your optometrist and optician. If you have Prism in your glasses, it is essential to see a Licensed Optician or Optometrist to have your lenses fitted.
IMPORTANT: Why Your PD (Pupillary Distance) Isn't Enough
PD is the most oversold and overhyped measurement in eyewear. It is only one number. PD simply measures the distance between your pupils. That is it. It's important, but it is not the only measurement needed to make good glasses. Relying on it alone is the reason so many online glasses cause headaches, distortion, and returns. PD does not include height, tilt, frame fit, eye alignment, or how your vision behaves in real life. These are the measurements that actually matter.
For precise, comfortable vision, we also need:
Monocular PDs (each eye separately—not just one number)
Segment height for multifocals and progressives
OC height (optical centre position)
Pantoscopic tilt
Wrap angle
Vertex distance (crucial in high prescriptions)
Frame shape, thickness, and base curve
Your reading distance
Your lifestyle and work demands
None of this appears on your prescription, which is why two people with identical prescriptions can walk out with entirely different lens designs and different results.
This is the truth —> Online retailers never mention the measurements that actually matter because they cannot measure them at all. They cannot determine how the frames sit on your face, where the optical centres should be placed, how much the frame tilts or wraps, or what distance the lenses sit from your eyes. They simply cannot do it. So they focus on PD, the one thing you can measure yourself, and then claim it is close enough. It is not even close.
Common Questions About Your Eyeglass Prescription
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They are diopters—units that measure how much focusing power your lenses must provide.
A “+” means farsightedness; a “−” means nearsightedness.
CYL and Axis correct astigmatism.
ADD corrects near vision.
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Most astigmatism falls between ±0.25D and ±4.00D.
Smaller numbers are mild; higher numbers are more visually significant.
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It means you need +2.50 diopters of extra magnification for near tasks such as reading or phone use.
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Myopia: stronger than −6.00
Hyperopia: stronger than +4.00
Astigmatism: more than 3.00 diopters
These prescriptions typically require specialty lens design and precise measurements.
Why You Should Still Visit an Optician (Even If You Understand the Numbers)
Even if you understand every value on your prescription, creating a functional, accurate pair of glasses requires professional measuring, adjustment, and verification.
At Helio Optometry, our licensed opticians measure:
Your lens position in the actual frame you choose
The exact height needed for reading zones in progressives
Your natural posture and head alignment
The curvature and tilt of the frame on your face
How your eyes track together at different distances
All of this is essential for comfortable, distortion-free vision—especially for:
Higher prescriptions
Astigmatism
Progressive lenses
Computer glasses
Large or wraparound frames
This is why even a "perfect" prescription can still feel wrong when glasses are incorrectly measured or made.
Why Does My Eyeglass Prescriptions Have an Expiration Date?
Most people assume their prescription expires for administrative or insurance reasons. It is actually a medical safety issue.
Your vision changes gradually, and even small shifts in prescription can cause headaches, strain, depth perception issues, or driving discomfort.
Your eye health can also change without symptoms, including conditions such as glaucoma, retinal disease, diabetes-related changes, cataracts, and macular degeneration.
The expiration date ensures that your prescription reflects:
Your most current visual needs
Accurate and safe lens powers
Eye health that has been properly checked
In Alberta, prescriptions typically expire in one to two years, depending on age, risk factors, and medical history. The expiry date protects you by preventing out-of-date prescriptions from being used to make glasses that may no longer be appropriate or safe.
CAUTION: Contact Lens Prescriptions Are Not the Same as Eyeglass Prescriptions
A lot of people are surprised to learn that contact lens prescriptions and eyeglass prescriptions are not the same thing. They may look similar at first glance, but they are completely different and definitely cannot be swapped. Glasses sit a few millimetres in front of your eyes, while contact lenses sit right on the surface, so the power often changes, especially in stronger prescriptions.
Contact lens prescriptions also include extra measurements that glasses never need, like base curve, diameter, lens material, and the exact brand that fits your eye safely. During a contact lens exam, we check how the lens centres, how it moves, how comfortable it is, how much oxygen it allows through, and how your eyes respond to wearing it.
So while it might seem convenient to use one prescription for both, it simply does not work that way. If you wear both glasses and contacts, you will always need two separate prescriptions to keep your vision clear and your eyes healthy. But that is a whole topic of its own, and we will save it for another article.
Final Thoughts (and Why This Matters)
Your eyeglass prescription is a roadmap, not the finished product.
Understanding the numbers is empowering, but the art and science of creating great glasses goes far beyond SPH, CYL, and PD.
If you have questions about your prescription—or you want glasses that truly match your day-to-day life—our opticians at Helio Optometry in Edmonton are here to walk you through everything step-by-step.
Book an eye exam or a glasses consultation anytime. You'll see the difference—literally.
Disclaimer: The content provided in this blog post by Helio Optometry eye care clinic in West Edmonton is intended solely for informational purposes and does not replace professional medical advice, diagnosis, or treatment by a Licensed Optometrist. No doctor/patient relationship is established through the use of this blog. The information and resources presented are not meant to endorse or recommend any particular medical treatment or guarantee and outcome. Readers must consult with their own healthcare provider regarding their health concerns. Helio Optometry and its optometrists do not assume any liability for the information contained herein nor for any errors or omissions. Use of the blog's content is at the user's own risk, and users are encouraged to make informed decisions about their health care based on consultations with qualified professionals.
