Understanding Your Contact Lens Prescription

In order to get yourself contact lenses, you will need a contact lens prescription from your eye doctor. But as with any other medical form, a contact lens prescription can be a bit overwhelming to try and understand.

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How Do I Read My Contact Lens Prescription?

You’ll generally receive a physical copy of your prescription from your eye doctor, along with your contact lenses.

The written out prescription will include the details of several measurements. These include the diameter, base curve, and power. There will also be figures for cylinder and axis, if you have astigmatism, and addition and dominant figures if you have presbyopia.

Since you might need different prescriptions for each eye, the numbers for your left and right eye will be separate.

The prescription will also include the name of the contact lens brand you have selected, along with its expiration date.

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What Do All the Numbers on the Prescription Mean?

It's one thing to know what the prescription says. It's another thing entirely to comprehend what it's saying. 

OD & OS: These stand for your right and left eyes, respectfully. OD is an abbreviation of the Latin term for Right Eye: "oculus dexter", while OS is the same for the left eye, "oculus sinister.” If you see OU on your prescription, that stands for "oculus uterque", meaning that both your eyes have the same measurements.

Diameter (DIA): Measured in millimetres, diameter (DIA) refers to the distance across the surface of the contact lens. The diameter of the contact lens determines where the contact lens will sit on your eye. Usually, the measurement is between 13 and 15, and if it's incorrect, the contact lens will be uncomfortable and may scratch your eye.

Base Curve (BC): The base curve measurement determines the shape of the lens to properly fit the eye based on its curve. There will be a written number for this, and there may also be a descriptive word (flat, median, or steep).

Power/Sphere (PWR/SPH): This is the strength of your prescription, and will vary based on how far or nearsighted you are. If you have been found to be farsighted, the figure will start with a + and if you are nearsighted, it will start with a -. The power number starts from zero and goes up by .25 diopters at a time, with higher numbers meaning more vision correction is needed.

CYL: If you are getting toric lenses, this cylinder value, usually a number between -4 and +4, details the power required to correct your astigmatism.

Axis: If you have astigmatism, your prescription will have an axis. The axis is measured in degrees. An axis is needed to compensate for the non-spherical shape of the cornea. An axis indicates the direction in which cylinder power should sit.

ADD (Add Power): If you are getting bifocal or multifocal lenses, you may see this figure on your prescription. This indicated the additional magnifying power in portions of the lens.

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Contact Lens and Glasses Prescriptions are Not the Same

Contact Lens and Glasses Prescriptions are Not the Same

In case you wear both glasses and contact lenses, you might have wondered if your prescriptions are interchangeable. For instance, can you submit an eyeglass prescription for a prescription for contact lenses, and vice versa?

The answer is no. There are distinct differences between eyeglasses and contact lenses prescriptions, regardless of their common objective - to correct "refractive errors" that are preventing you from seeing clearly.

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Common Questions

The length of time a prescription remains valid may vary depending on the laws in your state, though in most states a prescription expires after one year. It’s important that your prescription is up to date so that it is of the proper strength for your eyes, as that can change over time If your prescription is expired, you will not be able to purchase more contact lenses until you receive an updated prescription.
In the United States it’s illegal for contact lenses to be sold without a prescription. Without a proper contact lens prescription, the lenses may not have the proper strength or the right fitting to ensure they can be worn safely and provide the vision correction you need.
Understanding Your Contact Lens Prescription
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Contact lens prescriptions may seem like a confusing mess of letters and numbers, but each of those is carefully calculated to make sure that the lenses you purchase and wear are just right for you and provide the vision correction you need. To eyecare professionals, they provide a very clear set of guidelines as your lenses are made.

If you have any other questions, or if you would like to make an appointment for an eye exam or contact lens fitting, you can contact at .

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