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What’s in a Prescription?

It seems like contact lenses and glasses would use the same prescription, but in actuality, the two prescriptions are quite different. Few patients check the numbers or notice any change during the exam process, but the prescriptions are not the same because of where the lens sits in relation to your eye.

The lens of your glasses should rest about twelve millimeters from your eye whereas contact lenses are placed directly on the surface of your eye. Why would this make your prescription different? Well, think about holding a magnifying glass out in your hand. When you hold it far away and look through, the view you see is much different than if you try to hold the magnifying lens up close to your eye. The same principle is in play when you consider glasses in contrast to contact lenses.

Not all patients see a vast difference in their contact/glasses prescriptions, but usually, the power used for contact lenses is reduced. In addition to the powers being different, your eye care professional will need some additional measurements to fit you for contact lenses because there are specifications needed to fit contact lenses appropriately that aren’t needed for glasses.

While contact and glasses prescriptions are different, some aspects of the lens prescriptions are similar. Both have a lens power, which is the measurement used to correct your “refractive error,” or the nearsightedness, farsightedness, or astigmatism that you have. It is common to have a different prescription in each eye, so your right eye and left eye are measured individually regardless of what type of eyewear you’re getting. More technical aspects of your prescription (the power and axis determined to correct astigmatism, for instance) may also be included in both the contact lens and glasses versions of your prescription.

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