What Happens If Your PD Is Wrong on Your Glasses?

When ordering prescription glasses, most of the attention naturally goes to the numbers written on your prescription.

But there's another measurement that can be important when your lenses are made: your pupillary distance, usually shortened to PD.

So what exactly does it do, and why does getting it right matter?

What is pupillary distance?

Your PD is the distance between the centres of your pupils, measured in millimetres.

It helps determine where the optical centres of your prescription lenses should be positioned in relation to your eyes.

You may have one overall measurement, known as a binocular PD, or separate measurements for each eye, known as monocular PDs.

Why does PD matter when making glasses?

Prescription lenses aren't simply pieces of material with the same optical effect across every point.

When your glasses are made, the lenses need to be positioned so that you're looking through the appropriate part of each lens.

Your PD helps with that positioning.

What happens if your PD is wrong?

If the optical centres of the lenses are positioned away from where they should be, you may experience unwanted prismatic effects.

How noticeable this is depends on several factors, including the size of the error and the strength of your prescription.

With some prescriptions, a small discrepancy may not be particularly noticeable. With others, accurate positioning can become more important.

Could an incorrect PD make glasses uncomfortable?

Potentially.

Depending on the circumstances, incorrectly positioned lenses may contribute to symptoms such as visual discomfort or eyestrain.

However, discomfort in new glasses can have many possible causes, so PD shouldn't automatically be assumed to be responsible.

Is PD part of your glasses prescription?

Not necessarily.

Your spectacle prescription describes the lens powers required to correct your vision, while PD is a dispensing measurement used when the glasses are made.

This is why you may receive a prescription that doesn't have a PD written on it.

Can you measure PD yourself?

It is possible to measure PD at home using a millimetre ruler and a mirror, or with help from another person.

Accuracy matters, so the measurement should be taken carefully.

If you're uncertain about a measurement, it's better to check it rather than simply guess.

Don't use an average number

You may encounter supposed "average" PD measurements online.

Your glasses, however, are being made for your eyes.

Using your own measurement is preferable to choosing a generic figure simply because it sounds typical.

Check before submitting your order

When ordering prescription glasses online, take a moment to check all the information you've entered before completing the order.

That includes your sphere, cylinder and axis where applicable, as well as your PD.

Pay particular attention to whether you're entering one binocular PD or separate right and left measurements.

A small number with an important job

PD may look like one of the simplest measurements involved in ordering glasses, but it helps determine how your finished lenses are positioned in front of your eyes.

Taking the time to provide an accurate measurement is one more step towards getting prescription glasses made correctly for you.