Why is my contact lens prescription a different strength than my glasses prescription?

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Many contact wearers notice that the numbers on their contact lens prescription are different from the numbers on their glasses prescriptions. While some patients may think this is a mistake, there is a very simple explanation: distance.

The distance at which a corrective lens sits from the eye causes a notable difference in the strength of the prescription. This can easily be demonstrated by taking one’s glasses and extending them further away from the surface of the eye. If you have a nearsighted prescription, you will notice the power of the lens becomes weaker as you move it further away, and a farsighted prescription becomes stronger as you move it further away. Because of this, it is important that glasses prescriptions be tested at a similar distance to where a frame sits from a patient’s eye. During an eye exam, your doctor uses a phoropter to present multiple lens options at approximately 12 millimeters from the surface of the eyes.

When fitting contact lenses, the prescription must be adjusted, because contacts are placed directly on the eye, which is 12 millimeters closer than eyeglass lenses.  For nearsighted prescriptions, this means that the minus power of the contact will need to be weaker than in a glasses prescription. For farsighted prescriptions, it means the plus power of the contact will need to be stronger than in a glasses prescription. This difference varies depending on the strength of the prescription and only affects prescriptions over four diopters.

From the eye doctors, opticians and staff at Rinkov Eyecare Centers. We’ve provided comprehensive eye exams, contact lenses, eyeglasses, sunglasses and medical treatment of eye disease such as cataract treatment in Columbus, Ohio, for over 35 years. Come see an optometrist at one of our Columbus locations – Downtown, West, East, Westerville, Dublin, Bexley, Worthington and Nationwide Plaza.

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