What do the numbers on my eye prescription mean?
After an eye exam, you receive a prescription that looks something like this:
| Eye | SPH | CYL | Axis | Add |
|---|---|---|---|---|
| OD (right) | -2.00 | -0.75 | 180 | |
| OS (left) | -1.75 | -0.50 | 175 |
Each value describes how your eyes focus light and what correction is needed to give you clear, comfortable vision. Here is what they mean.
What is OD and OS?
- OD (oculus dexter) = your right eye
- OS (oculus sinister) = your left eye
- OU (oculus uterque) = both eyes together
These Latin abbreviations are standard on prescriptions in Canada and the U.S.
What is sphere (SPH)?
Sphere is the main correction for how your eye focuses.
- Negative values (e.g., -2.00) mean you are nearsighted (myopic) — distant objects are blurry
- Positive values (e.g., +1.50) mean you are farsighted (hyperopic) — close objects may be blurry
- The larger the number, the stronger the correction
A value of 0.00 or plano means no sphere correction is needed for that eye.
What are cylinder (CYL) and axis?
These describe astigmatism — when the front surface of your eye (the cornea) is shaped more like a football than a basketball.
- Cylinder is the amount of astigmatism correction
- Axis (1 to 180 degrees) indicates the angle of the irregularity
Not everyone has astigmatism. If your cylinder is blank or zero, your cornea is evenly curved and no astigmatism correction is needed.
Mild astigmatism is very common and usually nothing to worry about. For more on this, see Can I wear contacts with astigmatism?
What is add power (ADD)?
Add power is extra focusing strength for reading and close work. It appears on prescriptions for people with presbyopia — the gradual loss of near focusing ability that typically starts in your mid-40s.
- A typical add ranges from +0.75 to +3.00
- It is the same for both eyes in most cases
- It is used in progressive lenses, bifocals, and multifocal contact lenses
If you do not have an add value, your eyes can still focus at near distances without extra help.
For more on presbyopia, see What is presbyopia and how is it managed?
What is prism?
Prism corrects eye alignment problems. If your eyes do not naturally point at the same place, prism shifts the image so your brain can merge what both eyes see without strain or double vision.
Prism is measured in prism dioptres and has a direction:
- Base in or base out (horizontal alignment)
- Base up or base down (vertical alignment)
Most people do not need prism. If it appears on your prescription, your optometrist will explain why.
Is my glasses prescription the same as my contact lens prescription?
No. Contact lenses sit directly on your eye, so the prescription is different from glasses in several ways:
- Power may be adjusted because the lens is closer to the eye
- Base curve and diameter are added — these describe the lens shape and size
- A contact lens fitting is required to determine the right lens for your eye
You cannot use a glasses prescription to order contacts, or vice versa.
Can I take my prescription somewhere else?
Yes. In Ontario, your prescription belongs to you. You are free to fill it at any optical provider — the clinic where you were examined, an online retailer, or another shop. Your optometrist should provide it without conditions.
At Spadina Optometry, we always provide your prescription and are happy to answer questions about it, whether you fill it with us or not.
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Have questions about your prescription?
We take time to explain your results at every visit. If something is unclear, we are happy to walk through it with you.
Prefer to talk first? Call or text us at 416-703-2797.
Last reviewed: February 24, 2026