The Power of the Contact Lens Exam

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Glasses Prescription vs. Contact Lens Prescription

Have you ever wondered why your glasses prescription is different from your contact lens prescription?

Or if you’ve never noticed the difference, have you ever thought that you could just use your glasses prescription to get contact lenses?

Well, we’re here to let you know that your glasses prescription and contact lens prescription are not the same thing. Because glasses are positioned approximately 12 mm from the eyes while contact lenses rest directly on the surface of the eyes, this causes a discrepancy on the effective optics of the lens.

Consequently, using a glasses prescription to get contact lenses can cause eye strain and headaches due to the difference in power.

Thus, it is important to have a contact lens exam in addition to the regular comprehensive eye exam performed by your optometrist.

These two exams are separate and different, as a contact lens exam also includes a fitting evaluation and follow-up.

The contact lens exam

Even though there are multiple calculators and charts available online to convert your glasses prescription to a contact lens prescription, the contact lens exam is crucial to ensure that the right type of contact lens is chosen for you based on the unique features of your eyes.

The prescription is definitely not the only factor when it comes to choosing contact lenses. Contact lenses have additional specifications that glasses do not, such as base curve, diameter, material, and modality.

The base curve, or back curvature of the lens, helps the contact lens fit properly on your cornea, which can be flatter or steeper than others.

The diameter of the lens is important for lens centration and comfort; the wrong diameter can cause irritation and even abrasions.

The dryness of your eyes can help determine the appropriate water content of soft contact lenses, which are generally made of flexible, water-loving plastics called hydrogels.

In addition, the best modality for your eyes, whether they be daily disposable, 2-week replacement, or monthly replacement contact lenses, may differ from that of someone else based on your eyes and lifestyle.

As a result, changing brands or even just one of the aforementioned factors within a brand requires a contact lens exam by an optometrist. This even applies for plano, or “non-prescription,” colored contact lenses.

Visit your optometrist

Because contact lenses are a medical device, categorized as such by the FDA, they do require an eyecare professional to perform a contact lens exam, even if you only wish to obtain your prescription.

The use of improperly fitting contact lenses puts you at risk for infection, ulcers, and other complications. So the next time you want to order contact lenses, remember to see your local optometrist!

We highly encourage you to not use your glasses prescription as your contact lens prescription since the right contact lenses for you is dependent on so much more than just the power of the lenses.

Where is Insight Vision Center Optometry?

Serving the communities of Costa Mesa, Irvine, Newport Beach, Tustin, Santa Ana, Huntington Beach, and Fountain Valley. Schedule a visit with Dr. Valerie Lam or Dr. Thanh Mai to learn more. Our optometry practice is located in Costa Mesa at 3151 Airway Ave. Suite J2, Costa Mesa, CA 92626. We offer vision therapy and orthokeratology services.

About the Author:

Insight Vision Center Optometry is your friendly neighborhood office, utilizing the latest technology and a brand new office bursting with enthusiasm to give you the best vision. We service those in the Costa Mesa, Newport Beach, Santa Ana, Huntington Beach, Irvine, and Tustin areas extending throughout Orange County.


  1. Lillian Schaeffer July 28, 2016 at 6:38 pm - Reply

    This is some great information, and I appreciate your suggestion to visit an optometrist to get a contact lens prescription. I’ve had glasses for a little while now, but I’ve decided that it would be nice to have contacts for a few different activities. I’ll definitely look into visiting an optometrist to get a prescription so I can make sure the lenses fit properly. Thanks for the great post!

  2. Jack Mulligan August 11, 2016 at 4:47 pm - Reply

    I didn’t know that there was a specific contact lens exam. I usually get glasses because I’m a little wary of putting things in my eyes, but I’m getting sick of breaking frames. I’ll have to see if my optometrist can do a lens exam for me next time so I can try them out.

  3. Helen August 12, 2016 at 4:44 pm - Reply

    I still have not made the switch to contact lenses because I am really comfortable with my glasses. However, every time I play sports or go diving, I am more and more tempted to try out contacts. You mentioned that an eye exam for contact lenses is much different from other exams because they have to measure curve and diameter, but I would love to know how they do so.

    • Dr. Valerie Lam January 27, 2017 at 6:45 am - Reply

      Hi Helen!
      We have automated machines that can measure the curvature of your cornea, called the “K'” readings. This will help us to select the appropriate contact lens that will fit more appropriately. Since soft contact lenses are flexible, they are more lenient when choosing the right base curve. For rigid gas permeable lenses (hard contacts) we have to choose the exact lens that matches the curvature of your eye otherwise it will be uncomfortable and unsafe. We use special dye and blue light to evaluate for a perfect fit. Hope that helps!

  4. Dave Anderson January 10, 2017 at 6:34 pm - Reply

    I had no idea that there was a difference in prescription between the glasses and contact lenses. Now that I know that glasses and contact lenses have a different prescription I will make sure to get both prescriptions just in case. Also, I believe that glasses are a good option to avoid eye sensitivity that can come from contact lenses.

  5. Gina Henrie September 10, 2017 at 4:24 pm - Reply

    I have been looking to get some contact lenses or glasses but never really knew if they were the same thing, I’m glad I read what you said that the prescription is not the same thing. I want to make sure to get the right thing, thanks for your advice!

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