Key Takeaways
- Glasses sit away from your eyes, while contacts rest directly on them, so each needs its own measurements.
- A glasses prescription lists sphere, cylinder, axis, and sometimes an add for reading.
- A contact lens prescription adds base curve, diameter, brand, and material for a proper fit.
- Online calculators skip health and fit checks, so they can leave your eyes at risk.
- A contact lens exam includes an eye health check, a fitting, and follow-up care.
You grab your glasses off the nightstand, slide them on, and the world sharpens. Our team at Forest City Optometry helps you understand why contacts call for a different approach. They rest right on the surface of your eye, moving with every blink.
No, a contact lens prescription and a glasses prescription are not the same, and you can’t swap one for the other when ordering. Each one uses different measurements built for how the lens sits.
How Contact Lens and Eyeglass Prescriptions Differ
That small gap between a lens and your eye changes everything about how the numbers are figured out. Your glasses sit about 12 millimetres away, so the math behind the power shifts once a lens touches your eye directly.
What You’ll Find on a Glasses Prescription
Your glasses prescription looks like a short grid of letters and numbers. Each one tells the lab how to shape your lenses.
The sphere number sets the main lens power that corrects nearsightedness or farsightedness. If you have astigmatism, you’ll also see a cylinder number and an axis, which handle the uneven curve of your eye and point the correction in the right direction.
Some prescriptions include an add value. That extra power sits in the lower part of progressive lenses or bifocals, helping you read a menu or your phone without swapping glasses.
What Makes a Contact Lens Prescription Unique
A contact lens prescription carries everything glasses need, plus details tied to the shape of your eye and the lens itself.
Extra Measurements for Fit
The base curve tells the lens how much to bend so it matches the curve of your eye. Get it wrong and the lens feels tight or slides around.
The diameter sets how wide the lens is, so it covers the right part of your eye. Your prescription also names a specific brand and material, since each one fits and breathes differently.
Why Power May Shift
The power on your contact prescription can look different from your glasses number. That happens because the lens rests on your eye instead of sitting in front of it.
- The distance from your eye changes how much power the lens needs.
- Stronger prescriptions often need a bigger adjustment to match.
Why You Can’t Convert One Prescription to the Other
Plenty of websites promise to turn your glasses numbers into a contact order. The problem is what they leave out.
Online calculators often skip the base curve, diameter, and fit details that keep a lens comfortable and safe. They also can’t look at your eye. A contact sits on delicate tissue, and a poor fit can scratch the surface or block oxygen, which is why proper lens care matters so much.
A separate contact lens exam checks how your eyes handle a lens before you wear one all day. That step protects your vision in ways no calculator can match.
What to Expect at a Contact Lens Exam
A contact lens exam builds on a regular eye exam, then adds a few steps focused on fit.
The visit feels a lot like a routine appointment, with a little extra time set aside to get the fit right. You’ll try lenses on, see how they settle, and get comfortable putting them in and taking them out before you head home. Here’s what a visit can include:
- An eye health check and vision review.
- A fitting with measurements of your eye shape.
- Trial lenses to test comfort and clarity.
- Guidance on handling and cleaning.
- A follow-up to adjust the fit if needed.
Come See Us
Forest City Optometry offers contact lens exams along with family eye care, dry eye treatment, and myopia control at our London, Ontario, location just north of Huron on Adelaide. Book your eye exam online today and get fitted for lenses that match your eyes.
