Like an eyeglass prescription, a contact lens prescription includes the lens power required to correct your refractive error — whether 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 than those on your glasses prescription to give you the best vision possible.
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 mm, and the diameter of rigid gas permeable (GP) contacts ranges from 8.5 to 9.5 mm.
Lens brand or 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 Rx is valid for one year. You will need to revisit your eye doctor when your prescription expires, for a checkup of the health of your eyes before you can purchase additional lenses. Eyeglass prescriptions are regulated under state law, and most expire after two years.
You are entitled to a copy of both your glasses and contact lens prescriptions. It is illegal for your doctor to withhold your prescription from you.
You can request a copy of your glasses prescription at the conclusion of your comprehensive eye exam. But a contact lens prescription cannot be written by your eye doctor and given to you until he or she performs a contact lens fitting or has access to your previous prescription and has evaluated the fit of your current lenses.
Not everyone who needs eyeglasses can wear contact lenses successfully. Conditions such as dry eyes or blepharitis can make contact lens wear uncomfortable or unsafe. Even with no pre-existing eye conditions, some people have sensitive corneas and simply cannot adapt to contact lenses.