Contact Lens Prescription: What the Numbers Mean in Québec

In Québec, there is no separate prescription for contact lenses. Your optometrist gives you a single prescription for ophthalmic lenses, and it covers both glasses and contacts. The professional who then fits your contact lenses chooses their base curve, their diameter and the lens model, and is responsible for that choice.

Is my glasses prescription enough for contact lenses?

In Québec, it is the same document. The Ordre des optométristes du Québec (OOQ) says the regulations provide for only one prescription for ophthalmic lenses, meaning lenses with power, whether they are mounted in glasses or worn on the eye (our translation).

That prescription alone is not enough to order lenses. According to the Canadian Association of Optometrists (CAO), you need a valid prescription to start a contact lens fitting; once the fitting is complete, the contact lens details are set and you can buy your lenses from a licensed dispenser. Our article on wearing contact lenses for the first time explains how people get used to them.

What do OD, OS, SPH and CYL mean?

They are the eyes and the powers of the correction. According to the American Academy of Ophthalmology (AAO), OD (oculus dexter) is your right eye and OS (oculus sinister) your left eye. The sphere, SPH, is the lens power: a minus sign means nearsightedness, a plus sign farsightedness.

The cylinder, CYL, is the amount of astigmatism, if you have any. Our article on contact lenses for astigmatism explains how a toric lens corrects it.

What do AXIS, ADD, BC and DIA mean?

The axis and the add complete the power; BC and DIA describe the lens itself. According to the AAO, the axis, written in degrees from 1 to 180, shows which way the astigmatism lines up. The add, ADD, is extra lens power, for example for reading after age 40.

The CAO glossary defines the base curve, BC, as the curvature of the central part of the back of the lens, the side that touches your eye. DIA is the lens diameter. In the United States, the FDA says a contact lens prescription also names the brand, the power, the base curve and the diameter; in Québec, those details are set at the fitting.

Why isn't the base curve on my prescription?

Because that choice belongs to the person who fits the lenses. The OOQ says the optometrist writing the prescription should avoid listing the base curve, the physical dimensions of the lenses, the lens material or brand names.

The OOQ adds that it is up to the other professional, usually another optometrist or a licensed optician, who will fit and sell the contact lenses, to set those details and take responsibility for them. Québec's Dispensing Opticians Act says the same thing: an optician fits and sells ophthalmic lenses on the prescription of a physician or an optometrist.

Why are my contact lens numbers different from my glasses?

Because a contact lens sits on your eye, while a glasses lens sits a little in front of it. According to the AAO, that gap, called the vertex distance, is the main reason: a nearsighted person's contact lens usually has a lower minus number, and a farsighted person's may have a higher number. The AAO's example: -10.00 glasses may match a -9.00 contact lens.

The AAO gives another reason, monovision: one lens corrects one eye for distance and the other for reading, an option some people who wear bifocal glasses choose. The reading eye's lens then has a different number from the glasses.

What happens at a contact lens fitting?

A fitting checks that a lens corrects your vision well and suits your eyes. According to the Centers for Disease Control and Prevention (CDC), your eye care provider performs a complete eye exam, tries lenses on your eyes, checks how they fit, tests your vision through them and gives you instructions on wear and care.

The FDA says the choice of lens should be based on an exam by your eye care professional, not over the phone. The OOQ notes that a regular eye exam assesses whether you can wear contacts, but the tests that set the lens details are done at the fitting.

Why is there a follow-up visit after the fitting?

To see how the lens behaves after a few days of wear. At PrimeVision, the follow-up after a first fitting usually takes place a few days to a week later. The CAO advises wearing your contacts only for the length of time recommended, even if they feel comfortable, and having regular eye exams.

Our article on how many hours a day you can wear contacts explains how to follow the schedule you get at the fitting.

Can I get my prescription to buy lenses somewhere else?

Yes. The OOQ says an optometrist must respect the patient's choice of where to fill their prescription and must offer to hand it over at the end of the visit. The OOQ confirms this also applies when the patient wants to buy contact lenses.

If the optometrist recorded the details of contact lenses they fitted for you before, you can get a copy of that part of your file, the OOQ adds. A copy of the file is not a prescription, though.

How long is a prescription valid?

It is up to the optometrist's judgment. The OOQ says the validity period must be justified by a condition noted in your file and, for ophthalmic lenses, should normally not be less than one year, unless there is a serious clinical reason.

The FDA advises against ordering lenses with an expired prescription or stocking up right before it expires: it is safer to have your eyes checked. To know how often, read why eye exams are important.

Where can you get contact lenses fitted in Montréal?

At PrimeVision, 9520 Boulevard de l'Acadie, Suite 222, in Montréal (Ahuntsic), optometrists and a licensed optician work under one roof. The clinic fits silicone hydrogel soft contact lenses. Most trial lenses are available the same day when in stock, otherwise in 2 to 5 business days, and the follow-up after a first fitting usually takes place a few days to a week later. If a soft toric lens cannot correct your astigmatism, the clinic refers you to another professional. RAMQ covers eye exams for children under 18 and adults 65+.

Our page on contact lenses in Montréal covers fittings and trial lenses. For an up-to-date prescription, book an eye exam.

Sources

• Questions fréquentes : droits du patient et obligations professionnelles, Ordre des optométristes du Québec (in French).
• Eyeglass and Contact Lenses Prescribing and Dispensing Regulations by Province, Canadian Association of Optometrists, October 2024.
• Glossary of Common Ophthalmic Terminology, Canadian Association of Optometrists.
• Wear and Care for Contact Lenses, Canadian Association of Optometrists.
• Dispensing Opticians Act, CQLR c. O-6, sections 8 and 9, LégisQuébec.
• How to Read an Eyeglasses Prescription, American Academy of Ophthalmology.
• Is it possible for a glasses prescription to differ from a contact lens prescription?, American Academy of Ophthalmology.
• About Contact Lens Types, Centers for Disease Control and Prevention, 2025.
• Buying Contact Lenses, U.S. Food and Drug Administration.
• Contact Lens Risks, U.S. Food and Drug Administration.