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Are contact lens and spectacle prescriptions the same?

Doctor writing out a prescription

Contact lens prescriptions and spectacle prescriptions are not the same. They are significantly different because eyeglass lenses are positioned approximately 12 millimeters from your eyes and contact lenses rest directly on the surface of your eyes.

If you want to wear both contact lenses and eyeglasses, you will need two separate prescriptions.

What's the difference between a contact lens and glasses prescription?

Like a spectacle prescription, a contact lens prescription includes the lens power required to correct your refractive error — whether it's myopia (nearsightedness), hyperopia (farsightedness) and/or astigmatism.

But, depending on the degree of your refractive error and the type of contacts prescribed, the powers specified on your contact lens prescription may be significantly different from those on your glasses prescription.

Also, a contact lens prescription contains additional specifications that are not included on a glasses prescription, and these can be determined only after a comprehensive contact lens exam and fitting. They include:

  • Base curve. This measurement (abbreviated BC) is the curvature of the back surface of the contact lens. The proper base curve is determined by the shape of your cornea and produces a fitting that is not too loose or too tight.

  • Diameter. The lens diameter (DIA) specifies the overall size of the lens and, along with the base curve, determines how the lens fits. In most cases, the diameter of soft contact lenses ranges from 13.5 to 14.5 millimeters, and the diameter of rigid gas permeable (GP) contacts ranges from 8.5 to 9.5 millimeters.

  • Lens brand and material. The lens brand and material also must be specified on a contact lens prescription, because each lens material has a specific degree of oxygen permeability ("breathability"). This is especially important if you want extended-wear contact lenses or you occasionally fall asleep while wearing your contacts.

  • Expiration date. Generally, a contact lens prescription is valid for one year. You will need to revisit your eye care professional when your prescription expires to check the health of your eyes before you can purchase additional lenses. Spectacle prescriptions are typically valid for one to two years, depending on your age and various medical conditions.

You can request a copy of your spectacle prescription at the conclusion of your comprehensive eye exam. But a contact lens prescription cannot be written until your eye care professional performs a contact lens fitting or has access to your previous prescription and has evaluated the fit of your current lenses.

Can anyone get a contact lens prescription?

No, not everyone who needs eyeglasses can wear contact lenses successfully. Conditions such as dry eyes and blepharitis can make contact lens wear uncomfortable and/or unsafe.

Even with no preexisting eye conditions, some people have sensitive corneas and simply cannot adapt to contact lenses.

A note about coloured contact lens prescriptions

A contact lens fitting and a prescription written by a licensed eye care professional are required even if you have no need for vision correction and want only "non-prescription" (plano) coloured contact lenses or special-effect contact lenses to change the appearance of your eyes.

A contact lens, regardless of whether it is used for vision correction or cosmetic purposes, is a medical device, and should only be purchased from a licensed retailer with a valid prescription.

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