Why Your Eyeglass Prescription Is Only Half the Story
Every week, clients walk into my Calgary shop with a prescription in hand—often with an Add power of +1.50 or higher—and are told by their optometrist they need progressive lenses. That may be true for some, but not all.
As a licensed optician, I believe understanding how to read your glasses prescription is important—but more importantly, you need to understand how it applies to your daily life. Optometrists are excellent at diagnosing and prescribing. But when it comes to choosing the right lens design, that’s where an optician becomes crucial.
We don’t just look at numbers—we look at how you use your eyes.
What Do the Letters and Numbers Mean?
Your eyeglass prescription usually looks like this:
| OD (Right Eye) | -2.00 | -0.50 x 180 | +1.25 |
|---|---|---|---|
| OS (Left Eye) | -1.75 | -0.75 x 170 | +1.25 |
It includes:
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Sphere (SPH): Corrects nearsightedness (-) or farsightedness (+).
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Cylinder (CYL): Fixes astigmatism.
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Axis: Direction of the astigmatism (1–180°).
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Add: Additional power for close-up tasks like reading. Often for people over 40.
More complex prescriptions may include prism and base direction.
OD / OS / OU: Eye Identification
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OD = Right Eye
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OS = Left Eye
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OU = Both Eyes
You’ll see these at the start of your prescription to label which line is for which eye.
SPH, CYL, and Axis: What These Numbers Mean
Here’s a breakdown:
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SPH (Sphere): This is your lens power to correct nearsightedness (–) or farsightedness (+).
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CYL (Cylinder): This indicates astigmatism. If blank, you don’t have one.
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Axis: This tells us the orientation of the astigmatism. It’s a number between 1 and 180.
These three help shape the clarity of your distance or near vision.
What Is ADD Power and Do You Always Need Progressives?
ADD is short for “additional” power for reading. It’s typically used by people over 40 who need help seeing up close.
If your ADD is:
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+0.75 to +1.50 – You don’t need to jump straight to progressives.
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Consider anti-fatigue lenses or no-line bifocals instead.
Many optometrists prescribe progressives by default, but as opticians, we look at how you actually use your eyes:
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Are you mostly reading books?
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Are you on screens all day?
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Do you work at a fixed distance?
These are the types of questions I would ask to understand what lens is best suited for your lifestyle.
How to Read a Prism Correction in a Prescription
Prism correction is used to correct eye alignment issues, often related to double vision or muscle imbalance.
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Measured in diopters (e.g., 1.0△ or 2.0△)
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Usually prescribed in one or both eyes
You’ll also see directional terms:
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Base In (BI) – Prism moves image inward to help eyes turn out
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Base Out (BO) – Moves image outward to help eyes turn in
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Base Up (BU) – Moves image vertically up
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Base Down (BD) – Moves image vertically down
Do You Need Bifocal, Progressive, or Single Vision Lenses?
It depends on your lifestyle. Let’s break it down:
| Lens Type | Best For | Consider If |
|---|---|---|
| Single Vision | Distance, computer work, or reading only | You have an ADD power under +0.75 and don’t need multiple zones |
| Anti-Fatigue | Computer users, digital strain | You’re add power is between +0.50 and +1.25. Am not ready for bifocals or progressives. |
| Bifocals | Distance + reading, no intermediate zone | Your add powers are between +1.00 and +1.50 You don’t need computer distance correction |
| Progressives | Full range (distance, intermediate, near) | You’re comfortable adapting to multiple zones with add powers +1.50 and up |
Everyone’s eyes are different. Some people prefer using multiple single vision glasses over progressives, and that’s okay.
What matters is finding what works best for you.
My job is to guide you with the facts, so you can make the right choice with confidence.
Can You Reuse Old Frames With a New Prescription?
Yes. If your frames are in good shape, we can replace lenses in old frames without any compromise to visual clarity.
In fact, this is one of our most popular services for people upgrading to:
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Anti-reflective lenses
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High-index thin lenses
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New prescription changes
Still Confused by Your Prescription?
Bring it in. We’ll go through it line by line. And we’ll ask you questions your optometrist might not have asked:
What do you do all day?
What’s your screen time like?
Are you comfortable with lens switching or prefer all-in-one?
We don’t just follow numbers—we follow your lifestyle.
FAQ - How Do You Read a Glasses Prescription
Yes, +2.00 and +200 both refer to the same lens power in reading glasses. They each represent a strength of +2.00 diopters, used for near vision correction.
If your prescription includes an ADD number, you can convert it into a reading prescription by doing this:
Find the SPH (Sphere) and ADD values for each eye
Add the SPH and ADD together
That new number is your reading SPH
Keep the CYL (Cylinder) and Axis the same
Example:
OD: SPH -2.50, ADD +2.00 → Reading SPH = -0.50
OS: SPH -3.50, ADD +2.00 → Reading SPH = -1.50
That’s your new prescription for reading glasses.
A 0.75 prescription is mild.
You might see slightly blurry at a distance (if nearsighted) or up close (if farsighted).
Some people don’t need glasses.
But glasses can help with:
Driving
Reading signs
Reducing eye strain
If your eyes feel tired or you squint often, glasses may help.
The worst number is legal blindness, which means:
20/200 vision or worse in your better eye with correction
Or a visual field of 20° or less
In prescription terms, legal blindness often starts around -6.00 to -8.00 diopters or higher for nearsightedness.
But prescription alone doesn’t define blindness — it’s based on what you can actually see, even with glasses.
An optometrist or ophthalmologist can confirm if your vision meets this standard.
You can also explore these helpful resources if you want a second opinion:
