Why Do Your Glasses Prescription Numbers Change Over Time?

Look at two of your glasses prescriptions from different years and you may notice that the numbers aren't exactly the same.

Your SPH may have increased or decreased. Your CYL or AXIS might be different. One eye may even have changed while the other has remained relatively stable.

So why does a glasses prescription change?

Your eyesight isn't necessarily fixed

A spectacle prescription represents the lens powers required at the time your eyes are examined.

Your eyes can change throughout life, which means the prescription that gave you clear vision several years ago may not necessarily be the prescription you need today.

What does a change in SPH mean?

SPH, or sphere, represents the main lens power used to correct short-sightedness or long-sightedness.

A change in this figure means the spherical correction required by that eye has changed.

Remember that the plus or minus sign is important. It forms part of the prescription and shouldn't be ignored when comparing two prescriptions.

Can CYL change too?

Yes.

CYL represents the amount of cylindrical correction prescribed for astigmatism.

Your CYL value can change between prescriptions.

You may also find that a prescription that previously had little or no cylindrical correction contains one at a later examination, or vice versa.

Why has my AXIS changed?

AXIS describes the orientation of the cylindrical correction.

Because it works together with CYL, changes in the astigmatism correction can also result in a different AXIS measurement.

A higher AXIS number doesn't mean your eyesight has become worse. AXIS represents direction rather than prescription strength.

Can one eye change more than the other?

Absolutely.

Your right and left eyes don't have to change at exactly the same rate.

One prescription may remain quite stable while the other changes.

That's why each eye has its own set of measurements on your spectacle prescription.

Does a small numerical change matter?

It can.

Spectacle prescriptions are commonly written in relatively small power increments, so a difference that looks insignificant on paper may still represent a genuine change in the prescribed lens power.

Whether you actually notice that change will depend on the prescription and your individual vision.

Why does eyesight change with age?

The eyes naturally change throughout life.

Different changes can occur during childhood, early adulthood and later life.

From around middle age, changes to the eye's ability to focus at near also become increasingly common.

This is one reason somebody who had a stable distance prescription for years may eventually require different correction for close work.

Does a changed prescription mean something is wrong?

Not necessarily.

Prescription changes are common and can simply reflect changes in the focusing or refractive characteristics of your eyes.

However, sudden or significant changes in vision should be properly assessed rather than simply dealt with by repeatedly changing glasses.

Can you keep using your old glasses?

An older pair doesn't instantly become unusable because you've received a newer prescription.

How well you can see through them will depend on how much your prescription has changed and what you're using them for.

Your latest prescribed correction, however, is the appropriate reference when ordering new prescription glasses.

Don't combine numbers from different prescriptions

If you're ordering glasses online, use one complete current prescription.

Don't take the SPH from your latest prescription but use an AXIS or CYL from an older one because you preferred the previous numbers.

The measurements on a spectacle prescription work together.

Keep your previous prescriptions

Old prescriptions can still be useful records.

Keeping them allows you to see how your spectacle prescription has changed over the years.

Just make sure you don't accidentally use an outdated prescription when ordering your next pair.

Your prescription is a snapshot in time

The numbers written on your spectacle prescription describe the correction prescribed for your eyes at that point in time.

They aren't necessarily numbers you'll keep forever.

Understanding that prescriptions can change also explains why an old pair of glasses that once felt perfectly clear may eventually stop giving you the same quality of vision.