How to Read Your Glasses Prescription
A glasses prescription uses abbreviations that are easy to decode once you know what they mean. This guide explains OD, OS, SPH, CYL, AXIS, ADD, and PD in plain language.
Updated · Dr. David Wilkinson
Your optometrist is required to give you a copy of your prescription after a completed eye exam in Ontario. Here is exactly what each field means.
The abbreviations
| Abbreviation | Meaning |
|---|---|
| OD | Right eye (oculus dexter) |
| OS | Left eye (oculus sinister) |
| SPH | Sphere — overall refractive power |
| CYL | Cylinder — astigmatism correction |
| AXIS | Orientation of astigmatism (1–180°) |
| ADD | Addition — near-vision boost for presbyopia |
| PD | Pupillary distance (mm) |
SPH: nearsighted vs farsighted
The SPH value corrects your overall focus.
- Negative SPH (e.g., -2.50) = nearsighted (myopia). You see well up close, blurry at distance.
- Positive SPH (e.g., +1.75) = farsighted (hyperopia). Distance may be manageable but near vision is strained.
The further the number is from zero — in either direction — the stronger the correction required.
CYL and AXIS: astigmatism
Astigmatism means the cornea or lens is shaped more like a football than a perfect sphere. It causes blur at all distances.
- CYL is the power of the astigmatism correction. It can be negative or positive depending on the convention your optometrist uses (both are equivalent).
- AXIS specifies the angle at which the correction is applied, from 1 to 180 degrees.
If you have no astigmatism, CYL and AXIS will be blank.
ADD: for reading and progressives
The ADD value is the extra boost added to the lower portion of progressive or bifocal lenses so you can read comfortably. It is always a positive number (usually between +0.75 and +3.00) and typically the same for both eyes.
An ADD value appears once you have presbyopia — the natural age-related stiffening of the eye's lens that makes near focus progressively harder, typically beginning around age 40–45.
A sample prescription, decoded
OD (Right) OS (Left)
SPH -1.75 -2.00
CYL -0.50 -0.75
AXIS 180 175
ADD +1.50 +1.50
PD 32 32
This person is mildly nearsighted in both eyes, has minor astigmatism, and needs reading help (the ADD value) consistent with presbyopia. PD of 32/32 = 64 mm total.
Your right to your prescription
In Ontario, your optometrist must provide your prescription at the end of your exam at no extra charge. You can take this prescription to any optical retailer — you are not obligated to purchase glasses from your optometrist's office.
You can also ask your optometrist to include your PD measurement, which some offices charge a small fee to measure separately from the exam.
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Book an appointment →Frequently Asked Questions
- What does OD and OS mean on a glasses prescription?
- OD stands for 'oculus dexter,' the Latin term for right eye. OS stands for 'oculus sinister,' meaning left eye. Some newer prescriptions use RE (right eye) and LE (left eye) instead.
- What does a negative SPH number mean?
- A negative SPH (sphere) value indicates nearsightedness (myopia) — difficulty seeing clearly at a distance. A positive SPH value indicates farsightedness (hyperopia) — difficulty seeing clearly up close. The larger the number (ignoring the sign), the stronger the prescription.
- What is CYL and AXIS on a glasses prescription?
- CYL (cylinder) measures astigmatism — an uneven curvature of the cornea or lens that causes blurred or distorted vision at all distances. AXIS is the orientation of the astigmatism correction, expressed as a number between 1 and 180 degrees. If there is no astigmatism, CYL and AXIS will be blank or 0.
- What is ADD on a glasses prescription?
- ADD (addition) is the extra magnification added to the bottom of bifocal or progressive lenses to help with near vision. It is always a positive number and is typically the same for both eyes. An ADD value appears when presbyopia (age-related near vision loss) is present, usually starting around age 40–45.
- What is PD on a glasses prescription?
- PD (pupillary distance) is the measurement in millimetres between the centres of your pupils. It tells the lens lab where to centre the optical correction in each lens. Some prescriptions list a single PD (e.g., 64 mm) or separate values for each eye (e.g., OD 32 / OS 32). You can ask your optometrist to include your PD on your prescription.
Author
Dr. David Wilkinson, OD, FAAO — Pending clinical review
Optometrist, Spadina Optometry
A Toronto native and Fellow of the American Academy of Optometry, Dr. David Wilkinson has been part of our optometric practice since earning his OD from the Illinois College of Optometry in 2007. A certified clinical investigator with a special interest in contact lenses and myopia management, he serves as a part-time Clinical Instructor at the University of Waterloo's Waterloo Eye Institute and previously spent nearly a decade as Practice Advisor at the College of Optometrists of Ontario.