Lots of people wear glasses at their desk and switch to contacts for the gym or a night out. So it’s easy to assume one prescription covers both. We hear this question often, usually from someone who tried to order contacts online and hit a form asking for base curve and diameter.
Your contact lens prescription and your glasses prescription are separate, and you can’t use one in place of the other. Each one describes a lens that sits at a different spot in front of your eye, which is why you need a comprehensive eye exam for glasses and a contact lens exam for contacts.
Why Your Two Prescriptions Aren’t the Same
Glasses rest about 12 to 14 mm in front of your eyes, held in place by your nose and the arms of the frame. Optometrists call this gap the vertex distance. It changes how much power a lens needs to focus light on the back of your eye.
Contact lenses sit right on the surface of your eye, so there’s no gap to account for. Because the lens touches your eye, it has to fit the eye’s shape while also correcting your vision.
What’s on Your Glasses Prescription
A glasses prescription is shorter than most people expect. You’ll see a row for each eye, labelled OD for the right eye and OS for the left. Here’s what the numbers in each row mean:
- Sphere (SPH): The main lens power. A minus sign means you’re nearsighted, and a plus sign means you’re farsighted.
- Cylinder (CYL): The amount of correction for astigmatism, which happens when the front of your eye is shaped more like a football than a basketball.
- Axis: The direction of that astigmatism correction, written as a number from 1 to 180 degrees.
- Add: Extra power for close-up tasks like reading a menu or checking a text.
Not everyone has a cylinder, axis, or add value. If you’re over 40 and holding your phone at arm’s length, you’ll probably see an add on your next prescription.
What Makes a Contact Lens Prescription Different
A contact lens prescription includes power readings too, plus a few details about how the lens fits your eye. Each detail comes from measurements your optometrist takes during a fitting.
Base Curve and Diameter
Base curve describes how the lens is shaped so it can match the curve of your eye. Diameter sets how wide the lens sits across the surface. Get either one wrong and the lens can feel scratchy by mid-afternoon or slide out of place every time you blink.
Brand and Material
A contact lens prescription names a specific brand and material. Each brand has its own design, and the material affects how much moisture the lens holds and how much oxygen passes through it.
That’s why swapping brands calls for a new fitting, even when the power stays the same. Two lenses with identical numbers can feel very different on the same eye.
Adjusted Power
For people with stronger prescriptions, the power usually changes when moving from glasses to contacts. Since contacts sit closer to your eye, nearsighted people often need slightly weaker contacts, while farsighted people often need slightly stronger ones.
Astigmatism adds another step. Contacts that correct it, called toric lenses, are weighted or shaped so they stay in the right position each time you blink.
Can You Convert a Glasses Prescription to Contacts?
Neither prescription converts into the other. Your contact lens box won’t tell you what to order for new frames either.
Online conversion calculators only adjust the power. Your optometrist still needs to look at the surface of your eye and watch how a trial lens moves when you blink. Before you buy contact lenses online, make sure you have a current contact lens prescription from your optometrist.
Some eyes also need extra care before contacts feel comfortable. Dry eye and an irregularly shaped cornea can both affect your options, so your optometrist may suggest treating the dryness first or trying a different type of lens.
What to Expect at a Contact Lens Fitting
A fitting is a hands-on appointment. Most people go through a few stages before they settle on a lens.
Here’s what you can expect:
- An eye exam to check your vision and the health of the front of your eyes.
- Measurements of the curve and size of your cornea.
- Trial lenses so you can see how they feel.
- Practice putting lenses in and taking them out.
- A follow-up visit to check the fit after a few days of regular wear.
Take your trial lenses home and wear them through your normal routine. How they feel after a full workday tells your optometrist far more than how they feel after 10 minutes in the exam chair.
How Often to Update Your Prescription
Contact lens prescriptions usually expire after 1 year. Glasses prescriptions often last 1 to 2 years, depending on your age and eye health.
Don’t wait for the expiry date if something feels off. Book a visit if your contacts start feeling dry by dinnertime or your glasses don’t give you the same sharp view they used to.
Keep Your Contacts and Glasses Prescriptions Up to Date
Wearing both contacts and glasses gives you options for every part of your day, as long as each one comes with its own prescription. At Driftwood Optometry, we can update both prescriptions and help you find frames you’ll enjoy wearing on the days you skip the contacts.
Book your appointment and bring your current glasses and a contact lens box with you.
