How To Read Your Contact Lens Prescription

Similarities between contact and eyeglasses prescriptions

Contact lens prescriptions and glasses are not the same. However, you may recognize some abbreviations or text from your eyeglass prescription as you review your new contact prescription. Both glasses and contacts prescriptions contain information that helps identify the type of lenses and the power of the lens you need. Some of the abbreviations that appear in these prescriptions are the same. Some common terms you may be familiar with include:

• "OD" for "oculus dexter"

• "OS" for "oculus sinister" 

"OD" is the Latin term for the right eye and "OS" is the term for the left eye. Eyeglass and contact prescriptions often have the abbreviation "SPH" on them. "SPH stands for "spherical correction".

In some cases, "PWR" is listed instead of "SPH." "PWR" stands for "power." Both terms refer to the strength of the prescription. The number that's listed with these abbreviations will be measured in diopters, a term that refers to corrective lens strength. It will be preceded by a plus sign (+) if you're farsighted or a minus sign (-) if you're nearsighted.

If you have astigmatism — when the cornea, or lens of the eye, has an irregular shape, causing blurry vision — your prescription will include two additional abbreviations. One will be "CYL", which is short for "cylindrical power." This determines how strong a cylindrical lens must be to correct your astigmatism. 

Additionally, the term "Axis" will appear on the prescription, referring to the way that the cylindrical lens needs to be tilted to correct the astigmatism. If you don't need astigmatism correcting lenses, your eye doctor may write "DS," for "diopters sphere," meaning your eyes have a regular, spherical shape. Sometimes, an eyeglass or contact prescription may include information labeled "ADD," which is short for "additional correction." This is where details about bifocals, multifocal lenses or progressive lenses would appear.

Differences between contact and eyeglass prescriptions

Contact lens prescriptions contain extra abbreviations and numbers that don’t appear on eyeglass prescriptions. Luckily, our doctors are here to help make them easy to decipher. One of these new abbreviations is "BC," meaning "base curvature." This refers to the way that your cornea curves. It's necessary to include this detail because the shape of your contact lens needs to correspond to the shape of your cornea to ensure a good fit.

Contact prescriptions also include the term "DIA," which is short for "diameter," or the distance from one edge of the contact lens to the other. This is measured in millimeters. Additionally, your contact prescription should name the brand of contact lenses that your eye doctor prescribes, as well as an expiration date for the prescription. 

If your eye doctor gives you both eyeglass and contact prescriptions, you may notice some of the numbers listed alongside the corresponding abbreviations are different. This isn't a mistake; the prescriptions are different because people wear eyeglasses and contacts differently. Contacts sit on the surface of your eye, while eyeglasses sit a short distance away from your eye. Because you'll be looking through each lens from a slightly different distance, it's necessary to adjust the prescription accordingly. This ensures each type of corrective lens does its job from the appropriate distance.

How your eye doctor can help you understand your prescription

When you receive a prescription for contacts, your eye doctor should explain why they're prescribing a certain type of contact. For example, you may have an astigmatism that needs to be corrected. Or you may have a more complex prescription because you need contacts to help you see both near and far (multifocal lenses). Clarkson Eyecare provides several specialty contact lenses to help those with different eye conditions.

Additionally, the prescription includes information about the contact lens brand that your eye doctor recommends. They may recommend a certain brand of contacts for many reasons, including the shape and curvature of your cornea, the complexity of your prescription, whether you have dry eyes or other factors.

Your doctor should be interested to hear about why you want to wear contacts instead of, or in addition to glasses. Together, based on your prescription and other factors, you should be able to find contacts that are comfortable, fit well and help with vision correction.