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This post was most recently updated on November 7th, 2019
When you go to your eye doctor and get an exam done for contact lenses, your doctor should provide you with a paper copy of your prescription. This prescription contains various details from the power needed to correct your vision, measurements to assure the contact lenses fit right, as well as the specific brand of contact lenses that you have been prescribed.
It is important to keep in mind that contact lenses are considered medical devices by the Food and Drug Administration (FDA) and you should not purchase them without a prescription. Using contact lenses that are not properly fitted can lead to other serious complications. Here is some information to help you understand how to read a contact lens prescription, and what those measurements mean to you.
How to read your contact lens prescription
Have you ever wondered why your glasses prescription doesn’t work when you are trying to purchase contact lenses? Well, the reason is that when you purchase contact lenses, there are measurements for those contact lenses that are normally written on your contact lens prescription but not your glasses.
The power for your contact lenses may not be the same as your glasses prescription as well due to the contact lens sitting directly on your eye, and your glasses sitting a little bit further away from your eyes. Let’s take a look at what all these abbreviations mean so you have a better understanding of what is written on your prescription.
- OD – Everything that is on this line going horizontally is your prescription for your right eye. The abbreviation OD stands for “Oculus dexter,” which is the Latin term for “right eye.”
- OS Everything that is on this line going horizontally is your prescription for your left eye. The abbreviation OS stands for “Oculus sinister,” which is the Latin term for “left eye.”
- OU – Occasionally you might find this abbreviation on your prescription. The abbreviation OU stands for “Oculus uterque,” which is the Latin term for “both eyes”
- SPH – This is the amount of power that is needed to correct your vision. This measurement is measured in diopters and can correct nearsightedness as well as farsightedness. If the number has a minus sign (-) ahead of it then you are nearsighted. and if the number has a plus sign ahead of it then you are farsighted. The abbreviation SPH stands for “Sphere”, which means the correction is “Spherical”.
- CYL – If you have numbers under this column than your correction is not spherical, and the numbers indicate a lens power that is needed to correct astigmatism. If this column happens to be blank, then you’re in luck. You either have no astigmatism to correct, or it is so minor that your doctor didn’t feel the need for the correction in your contacts.
- AXIS – If your prescription has a number under the CYL column, then it must also have a number under the AXIS column. This measurement determines which orientation your lens needs to be in, in order for you to be able to see clearly.
- BC – This abbreviation stands for “Base Curve” and is the measurement of the back curvature of the contact lenses in millimeters. Your doctor will determine which base curve matches up with the front surface of your eye. If this measurement is incorrect, the contact lens could be too tight, or too loose on your eyes.
- DIA – This abbreviation stands for “Diameter” and like the base curve measurement it is written in millimeters. This measurement along with the base curve will determine how the lens fits on your eyes.
- LENS NAME – This is the brand of the contacts in which you have been prescribed. In the United States, your contact lens prescription must have a brand, and you are only able to purchase the brand that your doctor has fitted you. That doesn’t mean you can’t request your doctor to fit you for multiple brands of contacts. Occasionally you would be able to substitute a different brand that is made by the same manufacturer but marketed under a different brand.
- ADD – If you are wearing a presbyopia contact lens then you are going to have an “ADD” value on your contact lenses. Presbyopia affects your near vision, and this is the power that you need to be able to see up close.
- D & N – Also if you are wearing a presbyopia contact lens, you will find that your doctor wrote D and N on your prescription. This just indicates which eye is the dominant eye and which eye is the non-dominant eye.
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