How to Understand Power in Contact Lenses

If you wear contact lenses, you may be curious about some of the codes and numbers imprinted on the lens packaging. Optometrists prescribe contact lenses with different degrees of power to correct various vision problems. The Federal Trade Commission enforces rules that enable contact lens wearers to obtain their contact lens prescription to purchase contacts wherever they choose. Once you understand what the abbreviations and numbers mean, you will have a better understanding of your individual contact lens prescription.

Contact lenses have different powers to correct vision problems. (Image: lofilolo/iStock/Getty Images)

Step 1

Learn the basic Latin terminology that optometrists use on prescriptions. Ocular dexter or OD refers to the right eye and ocular sinister or OS refers to the left eye. If you are among the individuals with the same prescription for both eyes, you may see an OU for ocular uterque.

Step 2

Understand the sphere power of your lenses. Sphere power is the primary power in the lens to correct either hyperopia, farsightedness, or myopia, nearsightedness. Sphere power is positive if you are farsighted and negative if you are nearsighted. The unit of measure for sphere power is a dioptor, or D. Contact lens manufacturers make lenses in 0.25 D power increments. The greater the sphere power is from zero, the stronger the prescription.

Step 3

Consider the cylinder power if your prescription has one. Cylinder power is the power to correct astigmatism. This power also is in diopters and just like sphere power, the greater the number is from zero the stronger the prescription.

Step 4

Recognize the "add" power, if you wear bifocal lenses. Add power is an additional corrective power on part of the lens to assist with reading and viewing objects close-up. Add power is always positive.

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