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Contact Lens

Guide to Contact Lens Prescriptions

Contact Lenses Prescription Contact lenses are not one-size-fits-all. Even if you have perfect vision and only want lenses to change your eye color, you will still need to be examined by an eye care professional to get a prescription.

The same is true even if you already have a prescription for eyeglasses. You can't use a prescription for glasses to purchase contact lenses, because it does not contain all of the necessary information.

Getting a prescription for contact lenses
In the United States, the regulations for eye care professionals vary from state to state. In all states, you can visit an ophthalmologist or optometrist to be fitted for contact lenses, but opticians are only eligible for this certification in some states.

When you visit your eye doctor for a contact lens prescription, you can expect him or her to:

  • examine the overall health of your eyes to determine if you are a good candidate for contacts and, if so, which type of lenses would be best for your eyes.

  • determine the refractive error of each eye (that is, how much vision correction you need) and your degree of astigmatism, if any.

  • measure your eyes to see what size and curvature would be best for your lenses.

It is extremely important that you be properly fitted for contact lenses before wearing any. Ill-fitting lenses can be very uncomfortable and, more importantly, can cause damage to your eyes.

Understanding your prescription
You may feel intimidated when you first look at your contact lens prescription, but once you become familiar with a few standard terms and measurements, the meaning of that paper will become a lot more clear.

The following terms will be found on most contact lens prescriptions:

  • OS and OD - These two columns represent your left eye and your right eye. OD is an abbreviation for oculus sinister, Latin for left eye, OR stands for oculus dexter, meaning right eye.

  • Power (sometimes called Sphere) - This is the refractive power of the lens, measured in diopters. This represents the amount of vision correction that you need. A minus sign indicates a prescription for a myopic (nearsighted) patient, while a plus sign means a prescription for someone who is farsighted (hyperoptic). When no vision correction is needed, which is sometimes the case with color-changing lenses, this will say zero or plano (can be abbreviated pl).

  • Base Curve - The curve of the back of the lens, measured in millimeters.

  • Diameter - The diameter of the lens, in millimeters.

  • Brand - The brand of lenses being described. By law, all contact lens prescriptions in the United States must specify a brand, and retailers can only sell you the brand indicated.

These terms will only be used under certain conditions:
  • Cylinder and Axis - You will see these terms on your prescription if you have astigmatism. Cylinder is measured in diopters and represents the magnitude of your astigmatism. Similar to refractive power, a negative sign means myopic while a plus sign is hyperoptic.

    The axis is measured in degrees and represents the orientation of the cylinder that is needed.

  • Add Power - You will see this on prescriptions for bifocal contacts. It represents the power needed, in diopters, for reading or viewing close objects.

  • Color - This is seen only when special-effects lenses are being prescribed and indicates the desired color.

Where to purchase lenses
Thanks to a law passed in 2004, all patients in the U.S. are entitled to a copy of their contact lens prescription, so that they can purchase lenses wherever they wish. Good places to purchase lenses include local optical shops and warehouse stores, by phone, by mail order, and of course, through your eye care professional's office.

You can also purchase contact lenses from online retailers, but please note that all reputable retailers will require a valid prescription. It is illegal in the United States to sell contact lenses without a prescription, and any seller who does not care about the law, likely also doesn't care whether you are getting a safe, unexpired, FDA-approved product. You should also never buy contact lenses at a flea market or similar places.

Prescription expiration
Federal law mandates that your contact lens prescription be valid for at least one year, some states have laws that extend this period even further. Once your prescription has expired, you will be unable to purchase lenses until you have seen your eye care professional and obtained a new one.

It is important to visit your eye doctor after this one year period, instead of just stocking up on lenses beforehand. He or she will examine your eyes to make sure the lenses are not causing any damage. Even if you feel and see fine, there could be damage to your eyes that is only visible using a specific tool, such as a slit lamp. If not detected, this damage could increase over time, affecting your comfort and/or your vision, possibly irreversibly.

If you are having problems with your eyes or your lenses, you should see your eye doctor as soon as possible. Do not put it off because you are afraid you will be told you cannot keep wearing contacts; this is very rarely the case. Most likely, you just need a prescription for lenses of a different size, type, or material.

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