- Use a vertex distance conversion chart and note the difference in power between your glasses and contact lenses.
- If there’s a difference of 0.25 or less, you may be able to use a similar prescription for contacts.
- Contacts prescriptions differ from glasses prescriptions because they use the diameter and base curve of your eye within their measurements.
Converting a Glasses Prescription
Estimate the vertex distance to convert your glasses to contacts. Vertex distance measures the space between the back of a lens and the eye. Look at your glasses prescription and note the lens power. Then, find it on the above chart under one of the glasses power columns. If the difference in power between your glasses power number and the adjacent contact lens power number is less than 0.25, you might have a similar prescription in both glasses and contacts. This chart will get you close to your contacts prescription, but it may not be fully accurate. The best way to get your contacts prescription is to go to your optometrist. Unfortunately, this chart won't work if you have astigmatism because your cornea is shaped differently.
Are glasses and contacts prescriptions the same thing?
No, the prescriptions for glasses and contacts are different. Because glasses sit away from your eyes and contacts rest on your eyes, their prescriptions are not the same. Different measurements and tests go into both prescriptions, and using prescriptions interchangeably won’t give you the same visual acuity. Visual acuity measures how well an eye can recognize shapes and details at a given distance. Unlike glasses, contact lens prescriptions include the base curve and diameter of the eye, along with the needed lens material and brand/manufacturer.
What happens if you use the same prescription?
Using the same prescription will likely throw off your vision. For the most part, glasses and contact lens prescriptions should not be used interchangeably. Even if you wear contacts and glasses, your prescriptions can be very different from each other. The eye exams for glasses and contacts are also different, and the measurements and tests are not the same.
Contact Lens Prescription Specifications
Base curve The base curve determines the form and shape of your contacts. Optometrists will measure your eye to make sure the lens matches your eye’s shape and size. Basically, the base curve is the dome shape of the lens. A glasses eye exam and prescription doesn’t include this measurement because glasses sit away from your eye and don’t need to rest perfectly against your cornea.
Diameter The diameter is the overall size of the contacts. It measures how long the contact needs to be from edge to edge.
Lens material The lens material determines what your contacts are made of. Unlike glasses, contacts are made with various hydrogels. Some are made with strictly hydrogel, while others use a combination of hydrogel and silicone. Your optometrist will prescribe whatever best matches your needs. Hydrogel is a thin and malleable gel-like water-containing polymer that’s typically used to make disposable contact lenses. Silicone hydrogel lenses are ideal for dry eyes because they allow more oxygen into the eye.
Brand/manufacturer The brand/manufacturer states what company your contacts are from. An optometrist may recommend one brand of contacts over another for your specific needs. The manufacturer can play a big part in how a contact lens is made. Unlike glasses, contact lens brands are not equal. Different brands can have different base curves and diameters, so each prescription is specific to a given brand.
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