Steps to getting your contact lens prescription
Before you begin your purchase online, make sure you visit an optometrist or an eye doctor.
What to expect
Generally, a visit to the eye doctor includes an eye exam, a fitting, and a prescription. There are a few questions and concerns you should address in advance if you intend to use your prescription elsewhere. Before your exam, ask the doctor if they will prescribe a national brand. Some doctors will only prescribe private label lenses that are not available anywhere but from them.
Contact lens prescriptions are brand specific and cannot be substituted. National brands are available to be purchased anywhere, so requesting this in advance will make it easier to fill your prescription later. Doctors are legally required to provide a written copy of your prescription upon request. Be sure to ask for a copy each time you have an eye exam.
Eye exam & fitting
Once you have established that your doctor will prescribe a well-known brand, you will need an eye exam and a contact lens fitting. Contacts come in a variety of sizes, and the doctor will need to measure your eyes to get the right fit. The size will also depend on the type of lenses you want. The most common are soft lenses and RGP, or Rigid Gas Permeable lenses (read our RGP article here). Your doctor should discuss your lens options with you and help you determine which will best meet your needs. If you already wear contacts, but would like to try a different brand, it is best to have a new eye exam so you can be fitted for that brand.
Even if you just want to purchase cosmetic contact lenses that change your eye colour, you will still need an eye exam and a prescription from an eye doctor. The same applies to costume or theatrical contact lenses. However, you may not need a vision test to get a prescription for this type of lens – ask your eye care provider about their policy.
Can I use my glasses prescription for contacts?
Contact lens and glasses prescriptions are not the same. A contact lens must match the size and shape of your eye. A prescription for contact lenses contains information like base curve and diameter, as well as brand. Also, glasses rest about 12 millimetres from your eyes, while contacts sit directly on the eye. Contact lenses made to conform to a glasses prescription would be stronger than necessary, which could cause vision problems. Finally, your glasses are shaped to correct for astigmatism (irregular curve in the cornea or lens). However, contacts must be designed to fit the astigmatism, if there is one. If you have a prescription for glasses and would like to try contacts, visit your eye doctor for a new exam and contact lens fitting. Don’t forget to mention you will need a copy of your prescription.
How to read a contact lens prescription
A typical contact lens prescription looks like this:
- Eye – Refers to which eye the prescription line item is for. The OD (oculus dexter) means Right Eye, while OS (oculus sinister) refers to Left Eye.
- Power / SPH – Also referred to as PWR, this field references the corrective factor of the contact lenses. A negative number, such as -2.25, in this field indicates myopia, (nearsightedness), while a positive number like +2.25 indicates hyperopia (farsightedness). If your prescription shows the letters PL, which stands for Plano, it means the no correction is needed.
- DIA – Short for Diameter, this measurement refers to the size, in millimeters, from edge to edge of the contact lens. Soft contact lenses are larger than RGP lenses, but either way, the diameter usually falls between 13 and 15.
- BC – This stands for Base Curve, and is the diopter measurement, in millimeters, of the inside curve of your contact lenses. It is usually a number between 8 and 10 and is sized to match or correct the curvature of your eye.
- Used for bifocal contact lenses, the number here, measured in diopters, adds a degree of magnification to certain portions of the lens.
- CYL – This measurement, which stands for Cylinder, shows how much astigmatism, if any, you have. It is measured in diopters.
- AX – Short for Axis. Measured in degrees, the axis references the placement of the power in the lens, to compensate for the cornea’s shape when there is astigmatism.
Your prescription will also have an expiration date, usually one to two years from the date of the day it was given to you. Note that once your prescription has expired, you can no longer use it to buy contacts.
Prescription on your contact lens box IS NOT a written contact lens prescription
Your contact lens prescription
Upon completion of refraction, every qualified optician shall provide his or her patient with the following information and document it in the patient’s record: (a) results of refraction (eg. correction needed in dioptres) (b) the type and quantity of any optical appliance to be prescribed and/or dispensed; (c) the purpose and indications for use of any optical appliance to be prescribed and/or dispensed where applicable (d) referral to an optometrist, ophthalmologist or other specialists, if any, and reasons for the referral.
Contact lens prescription checklist
Now that you’ve had your exam and fitting and have been given your prescription, double check to make sure everything is in order.
- Make sure that you have been given a prescription for contact lenses and not for eyeglasses. A prescription for disposable contact lenses will always include a contact lens brand name.
- Also, verify that the prescription is filled out completely and accurately, with all the relevant information. A soft lens prescription usually includes a lens material name, design, power, base curve, and diameter.
- Once you’ve verified that all the information is correct, you are ready to make a contact lens purchase from the outlet of your choice.