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It seems like contact lenses and glasses would use the same prescription. After all, the idea is that your eyes don’t see quite right, so you use a lens to change the view, and then you see clearly, right? Well, 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 rests about twelve millimeters from your eye. Contact lenses, on the other hand, 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.
If you’re still imagining the magnifying glass held out in your hand, think about how grass would look if you’re sitting on the ground and have the magnifying glass down near the grass. You’d be able to see the blades clearly, right? If you held the magnifying glass up close to your eye, the refraction would be so strong that you wouldn’t be able to see anything other than a blur. The power of the glass would need to be reduced for you to see clearly. This is the reason why your contact lens prescription is weaker than your glasses, but you get the same crisp, clear vision with each. Neat, right?
Not all contact lens prescription powers are drastically less than the glasses prescription for the same person, 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.
Some things that need to be measured to receive an accurate contact lens prescription are the size of the cornea, the curve and overall size of the lens, and the suggested brand of lenses that will work best for you. Eye care professionals usually have an idea of which contact lenses work best with different eye conditions, so they will suggest a particular toric lens for your astigmatism, or a type of disposal lens based on your needs, for example.
Some aspects of the lens prescriptions are included for both kinds of eyewear. The lens power is included in both prescriptions. Lens power is the measurement used to correct your “refractive error,” or the nearsightedness, farsightedness, or astigmatism that you have. It wouldn’t be identical because of the magnifying glass analogy, but it is included in both prescriptions. They’re also laid out per eye—right vs. left—because very few people have the same prescription in both eyes, so they 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.
Hopefully that makes the difference between your contact lens and glasses prescriptions clearer. Talk to your eye care professional if you have any questions about your prescription or are curious about getting contact lenses or glasses.