How do you read a contact lens prescription? Your contact lens prescription is significantly different from your eyeglass prescription. Although some of the numbers and measurements are the same, a contact lens prescription has a series of fields that are not included on your eyeglass prescription. These fields signify important aspects, like the curvature and depth of the contact, to ensure that your contact lenses fit and function properly. For this reason, your eyeglass prescription cannot substitute for your contact lens prescription, and visa versa. Here are some of the fields and their definitions that you will see on your contact lens prescription: contact prescription

OD – Oculus Dexter, Latin for the right eye.

OS – Oculus Sinister, Latin for the left eye.

PWR – Power, the diopter measurement needed to correct your vision.

BC – Back Curvature, curvature needed in the contact lens to match the curve of your eyeball.

DIA – Diameter, the measurement of your contact from one end to another.

CYL – Cylinder, measurement of astigmatism (if applicable).

AXIS – Measures the shape of the eyeball to correct astigmatism (if applicable).

ADD – Diopter measurement needed for bifocal contact lenses.

How the terms work:

O.D. and O.S. are two separate fields on the prescription. By differentiating between the right and left eye, a doctor can write a separate prescription for each eye on one page. The rest of the terms below are used to describe the prescription for the left and right eye.

PWR is measured in diopters, and denotes the amount of correction needed to restore optimal vision. If the number is negative, it means nearsightedness. A positive number denotes farsightedness.

BC is necessary to ensure that the contact lens sticks to the eye and stays in place. With a correct BC measurement, the contact lens will fit safely and securely on the cornea, and the wearer won’t notice it’s there.

CYL is the lens power needed to correct astigmatism. Since astigmatism is the unfocused direction of the light to the retina, it causes the light to be off-center in various degrees.

AXIS is the degree of where the light hits your eye after entering the pupil. The AXIS and CYL work together to produce a contact lens that corrects the astigmatism. If you do not have astigmatism you will not see an entry for the AXIS or CYL field.

ADD is the diopter number needed for patients who want bifocal contacts. Since bifocal lenses have two different prescriptions, this number will be different than the PWR diopter.

Your unique contact lens prescription combines these features to design a pair of contacts that will be optically best for you. The prescription may also have an expiration date, brand name, and other fields, but most of these are optional by the doctor. By using these universal terms to describe ocular imperfections, prescriptions can be interpreted by eye doctors around the world.

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