What to Remember
- OD is your right eye, OS is your left eye, and OU means both.
- SPH corrects near or farsightedness: minus is nearsighted, plus is farsighted.
- CYL and Axis always appear together and correct astigmatism.
- PD (pupillary distance) is usually not on a Canadian prescription and is measured when your glasses are made.
- Always confirm your prescription is current with your optometrist before ordering.
What Is An Eyeglass Prescription Chart?
An eyeglass prescription chart is a small grid of your two eyes' correction values. Each row is one eye: OD (right), OS (left), and sometimes OU (both). The columns hold the measurements: SPH, CYL, Axis and Add, plus occasionally Prism and Base. Most values appear in diopters in 0.25 steps; the axis is a whole number from 1 to 180. These are the same values you will enter when ordering prescription glasses at BonLook.
Step-By-Step: How To Read Your Prescription Chart
Step 1 â Find Your Right Eye (OD) And Left Eye (OS)
Start with the rows. OD is oculus dexter, your right eye, and OS is oculus sinister, your left eye. The right eye is listed first by convention. OU means both eyes, and some Canadian prescriptions use RE and LE instead.
Step 2 â Read The Sphere (SPH)
The sphere (SPH) shows the strength of your near or farsightedness in diopters. A minus sign means you are nearsighted (myopia), so distant objects blur. A plus sign means you are farsighted (hyperopia), so close objects blur. The further from zero, the stronger the correction, so double-check the sign before ordering: a â2.00 and a +2.00 point in opposite directions.
Step 3 â Read The Cylinder (CYL) And Axis
The cylinder (CYL) column shows how much astigmatism correction you need; a blank column means none. When a cylinder value is present, it always travels with an axis number between 1 and 180 that describes the orientation of that correction. Astigmatism comes from an irregularly shaped cornea or lens and can blur vision at all distances. Always read the two numbers together.
Step 4 â Read The Add (And Prism, If Present)
The add is extra plus power for reading or multifocal lenses, usually the same for both eyes. It is tied to presbyopia, the age-related difficulty focusing up close that usually appears after age 45. You might also see a prism value with a base direction (BU, BD, BI or BO), a less common correction for double vision. If unsure why it appears, ask your optometrist.
Step 5 â Check For Pupillary Distance (PD)
Pupillary distance (PD) is the space between the centres of your pupils in millimetres, and it aligns the optical centre of each lens. Adults usually fall between 50 and 70 mm, with many around 63 mm. In Canada, PD is usually not printed on your prescription, because measuring it is the dispensing provider's responsibility. You can have it measured professionally or measure it at home, and BonLook walks you through PD when you order.
A Worked Example: Reading A Sample Prescription Chart
Reading it row by row makes the numbers click.
| Eye | SPH | CYL | Axis | Add |
|---|---|---|---|---|
| OD (right) | â2.00 | â | â | +2.00 |
| OS (left) | â1.00 | â0.50 | 180 | +2.00 |
Your right eye (OD) has a â2.00 sphere with no cylinder or axis, so it is mildly nearsighted with no astigmatism. Your left eye (OS) has a â1.00 sphere with a â0.50 cylinder at axis 180, so it is mildly nearsighted with slight astigmatism at the 180 meridian. Both eyes carry a +2.00 add, which powers the reading portion of a multifocal or progressive lens.
How Strong Is My Prescription?
There is no single 'good' or 'bad' number. The further your sphere sits from zero, the stronger the correction, meaning your eyes need more help focusing. According to the Cleveland Clinic, a sphere below â5.00 is considered high myopia, and more than +3.00 is high hyperopia. These are clinical classifications, not Canadian standards, and a high number does not mean anything is wrong. Only your optometrist can interpret your full prescription, so confirm it with an eye care professional before you buy.
From Chart To The Right Pair (Canadian Buyer's Notes)
Once you can read the chart, choosing the right pair gets easier. First, confirm your prescription is current: the Canadian Association of Optometrists recommends an expiry no longer than two years, and exam frequency shifts with age. You're also entitled to a copy of your prescription. In Ontario, optometrists must provide it in writing. A glasses prescription isn't a contact lens prescription, which needs its own fitting.
Next, match your numbers to lens choices: a single SPH value points to single-vision lenses, while an add suggests progressive lenses. At BonLook, single-vision and progressive prescription glasses start at $149 with lenses included, plus blue-light or UV-reactive options. Shipping is free on orders over $80, with free returns within 30 days. One note for Saskatchewan: purchasing corrective eyewear online is considered illegal there. Always confirm your prescription with your optometrist before purchasing.
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