Is your contact lens grade the same as your eyeglass grade?

You know your grade. It is written on the little card from your last eye test, and you have typed it into an online cart more than once. So can you order contact lenses in that same power? Usually not — and for stronger prescriptions, definitely not.

The 12 millimetres that change everything

Your eyeglass lenses sit about 12 millimetres in front of your eyes. A contact lens sits directly on the cornea. That gap is small, but it changes how much correcting power reaches your eye.

Move a lens closer to the eye and its effective power changes. So the number that works at 12 millimetres away is not the number that works at zero. Optometrists convert between the two using a calculation based on that distance, called vertex distance.

The direction of the change is consistent:

• If you are nearsighted (minus power), your contact lens power is usually slightly weaker than your eyeglass power. A -6.00 in glasses often becomes around -5.50 in contacts.
• If you are farsighted (plus power), it goes the other way — your contact lens power is usually slightly stronger.

Below about 4.00 in either direction, the difference is often small enough to round away. Above that, it matters, and it gets larger the stronger your prescription is. This is exactly why people with high grades who order contacts using their glasses numbers end up with vision that feels almost right but never sharp.

Your eyeglass prescription is missing information

Even with the power converted correctly, an eyeglass prescription does not contain everything a contact lens needs. A contact lens sits on living tissue, so it has to fit the shape of your eye:

• Base curve. How curved the lens is, matched to the curvature of your cornea. Too flat and it slides around. Too steep and it grips.
• Diameter. How wide the lens is across.
• Material and water content. Which decides how much oxygen reaches your cornea and how the lens behaves through a full day in air-conditioning.
• Brand and modality. Contact lens parameters are not interchangeable between brands. A prescription is written for a specific lens.

Astigmatism makes it more particular, not impossible

If your prescription has a cylinder and axis, you need a toric contact lens — one weighted to stay in a fixed orientation on your eye instead of rotating. Toric lenses work well, but they need to be fitted and then checked on your eye, because how the lens settles varies from person to person. Ordering them from paper numbers alone skips the part that determines whether they work.

Why the fitting is a separate appointment

A contact lens fitting is not the same as an eye test, and one does not replace the other. During a fitting, your optometrist measures your cornea, assesses your tear film, chooses a starting lens, then puts it on your eye and evaluates how it moves, centres and settles. You are taught to insert, remove and clean the lens safely. Then you come back so the fit can be checked while it is actually being worn.

That last step is the one people skip, and it is the one that catches problems early — a lens sitting too tight, a cornea not getting enough oxygen, a tear film struggling in a heavily air-conditioned office.

What this means if you already wear both

Keep both prescriptions current, and expect the numbers to look different. That difference is not an error. Have your eyes tested at least once a year, and sooner if your contacts feel less comfortable than they used to, if your eyes redden by afternoon, or if your vision fluctuates through the day.

And never wear contacts every waking hour. Your eyes need time in glasses, especially in a country where you move between hard sun, traffic dust and cold office air in a single commute.

Get both numbers, properly

Book a Vision7 eye test at your nearest Vision Express store and ask for a contact lens fitting alongside it. Our seven-step comprehensive eye exam establishes the prescription, and the fitting translates it into contact lens parameters that are measured on your eye rather than estimated.

While you are there, sort out the backup pair — the glasses you reach for on the days your eyes need a break. Ask for Essilor Blue Light Single Vision at the counter: a single-vision lens made for people who spend their day on screens, which is most of us by the time the contacts start feeling dry. If you want it the same day, ask about the Opulens Advanced 1.60 instead — in stock and ready in 20 minutes. Free lifetime cleaning and fitting adjustments either way.