What is PD, and Why Does it Matter?

When you visit your optometrist for a new pair of glasses, the optical dispenser will measure your pupillary distance (PD) as part of the fitting process. 

PD is the distance between the centre of each of your pupils. It can be measured using a small ruler (a PD rule!) or a device similar to double-ended binoculars with a light inside to create a reflection.

The PD measurement helps make sure the alignment between your pupils and your spectacle lenses is accurate, so you are looking through the optical part of the lens for best vision. 

Why the PD measurement is important

If the PD measurement is inaccurate, it could leave you looking off-centre in the lenses, potentially leading to eye strain, headaches, blurred vision and even seeing double. 

Having a correct PD measurement for your glasses aids in:

• Prescription accuracy: If the PD is not taken into account during the glasses fitting, the lenses may not be as effective and the prescription may not provide the proper correction. 

• Visual clarity: The point where the prescription is most accurate is the optical centre of a lens. With an accurate PD you are looking through the correct part of the lens, which minimises distortion and maximises visual clarity. 

• Proper binocular vision: Binocular vision is where both eyes work in unison to create a clear image. If the PD is wrong, you may see a double image.

• Adapting to bifocal or multifocal lenses: PD is particularly important in prescriptions with higher diopters (high powered glasses prescriptions), or specialised lenses like progressive (multifocals), extended readers or bifocals. An incorrect pupillary distance may lead to difficulties in adapting to bifocal and multifocal lenses. 

What factors affect the PD measurement?

PD varies depending on the type of prescription you have. The PD for distance correction glasses is different to reading correction glasses, simply because your eyes converge (turn inwards) to read, which makes the PD smaller, normally by between 2 and 4mm. 

The measurement can also vary depending on the frame you choose. You may have an uneven face or slightly crooked nose. The frames may sit slightly more to one side than the other. 

In this case, the optical dispenser will measure the PD of each eye individually (often called a mono PD) to make sure the lenses will still be centred in front of your eye, despite how the frame sits. 

What if I want to order glasses off the internet?

If you order glasses online, the website will ask you to enter your PD measurement. But it’s often better to visit an optometrist for accurate fitting in person. 

This is because your PD is not always printed on your prescription, and it’s difficult to accurately assess it yourself over the internet. If the frames you choose do affect the PD, the glasses may not be comfortable or effective.

Some websites, and all ready-made magnifiers you buy off the shelf, use an “average” PD measurement to make their glasses. Sometimes this isn’t a problem, but the more their PD is different from yours, or the higher power lens you have, the more likely it is that the glasses won’t feel comfortable to wear. 

If you need new glasses and have questions about PD, book an appointment. We’ll explain how PD works during your eye exam.