What’s the difference between a glasses prescription and a contact lens prescription?  Can one be used to order the other? This post looks at why the two prescriptions are different.

Can I order contacts with my glasses prescription?

No. Quite simply, you can’t. Your glasses prescription is just a starting point for a contact lens practitioner to determine your contact lens prescription, but it doesn’t tell the whole story. There are several reasons why the two prescriptions are not the same, which this post will explain.

Why is the power different between my glasses and contact lenses?

A glasses prescription provides information about the strength of the spectacle lenses that will correct your vision. With lower powers, the contact lens power will be similar in power to your glasses. But once the lens power goes above +4 or -4, the contact lenses will be a different strength to your glasses.

This is due to the optics of the lenses and the eye. In glasses, the lens sits a small distance in front of the eye. Contact lenses sit directly on the eye. Wearing a lens that is too strong or too weak for your eyes can lead to vision problems including eyestrain and headaches.

Glasses prescriptions usually have three numbers for each eye – being the power of lens needed to correct both curves of your eye as well as the orientation of the stronger power. We’ve previously written about this in Understanding your eye test results.

Why does my contact lens prescription not include my astigmatism?

Mild-to-moderate astigmatism is masked by wearing contact lenses. Soft contact lenses will mask small amounts of astigmatism and hard – or RGP – contact lenses will mask quite significant amounts of astigmatism.

This is due to the fact that a contact lens is like a shell on the surface of the eye. It creates a more regular surface on the eye and your tears fill in the This is another reason why the power of your contact lenses differs from your glasses.

Small amounts of astigmatism are not included in contact lens prescriptions. That’s why many contact lens prescriptions only have one number for the power of the lens.

What about the other numbers on my contact lens prescription?

Your contact lens prescription also contains at least two other numbers: the base curve and the diameter of the lens. Your lens practitioner will determine which base curve and diameter for your particular eyes.

Many disposable contact lenses come in only one choice of diameter and one or two base curves, meaning most of these lenses are identical in fit. But most eyes are not identical and one size does not actually fit all, despite the claims.

You should always have your contact lenses fitted by an optometrist – even cosmetic lenses, which are no different to other contact lenses when it comes to fitting.

My soft lenses correct my astigmatism but why are the numbers different to my glasses?

If you’ve got astigmatism of -0.75 or more and you wear soft contact lenses, the chances are that they are toric lenses. This means they are specially shaped to correct your astigmatism. They do this by being stronger in some parts of the lens than others, to match the power of your eye.

Glasses lenses also correct astigmatism by having two different curves. The lens is held firmly in place in the correct orientation by the glasses frame, so the vision stable.
But contact lenses float on a film of tears and are free to move and rotate on the eye.

Toric lenses are shaped to be thinner in some areas and thicker in others. Gravity as well as the squeezing mechanism of blinking cause the soft toric contact lens to orient itself a certain way in your eye.

The problem is, each eye is different and it is hard to predict how a lens will sit once it settles down in the eye.  This is why you often need to trial a toric lens so adjustments can be made before ordering. If the lenses rotate in towards your nose a little, due to your unique eye shape and blink, the axis of the astigmatism will need to be adjusted to account for this rotation.

This is one reason why the axis can be different in contact lens prescriptions compared to glasses prescriptions.

The other reason is that, unlike glasses lenses, soft toric contact lenses for astigmatism are often not available in every possible orientation, especially disposable lenses.

Your optometrist has to determine the closest fit and this may mean a different axis compared to your glasses.

If you order toric contact lenses based on your glasses prescription, chances are, your vision won’t be right.

Can I use my contact lens prescription to get glasses? 

Once again, the answer is no, for similar reasons to those explained above. An eye test will quickly determine exactly what your glasses prescription is so you can be prescribed the right lenses for your eyes.

Should I buy my contact lenses online?

In our digital age some contact lens wearers have gone online to order their lenses. Online retailers offer discounts on bulk orders and the convenience of ordering from home.  We’ve always advocated against this, since studies have shown incidences of contact-lens related eye infections is higher in people who order contact lenses online (1).

Don’t take chances with your eye sight. Talk to the experts. Call The Eye Practice on (02) 9290 1899 or make an appointment online today.

Reference

(1). The incidence of contact lens-related microbial keratitis in Australia.

Stapleton F et al.  Ophthalmology. 2008 Oct;115(10):1655-62.