What Is Pupillary Distance?
When placing your first order of prescription glasses online, you’ll likely come across some pretty specific language relating to the various measurements required.
If you’ve never actually had to read your prescription before, some of the terminology used on it will probably be new to you.
One of those terms is pupillary distance, which simply refers to the distance from the centre of your left pupil to the centre of your right pupil.
Pupillary distance is measured in millimetres and is crucial knowledge when fitting corrective lenses. We’ll take you through the ins and outs of it in this article.
Why is pupillary distance important?
When creating prescription lenses, having the wearer’s correct pupillary distance (PD) is vital, as it allows the lens maker to determine the centre of the lenses.
For the clearest and most comfortable vision possible, the distance between the centre of the left lens and the centre of the right lens should equal the distance between the wearer’s pupils.
Incorrect PD measurements, even if they’re only off by a millimetre or two, could be the difference between perfectly functioning prescription glasses and distorted lenses.
Each pupil needs to align as closely as possible with the centre of the lens directly in front of it in order to achieve optimal vision correction.
If the lenses are created with an incorrect PD, the wearer will experience distorted, uncomfortable vision, and possibly headaches as a result. In this case, the correct PD measurement would need to be taken so the lenses can be realigned.
DID YOU KNOW?
The ruler that eye care professionals use to measure pupillary distance is commonly known as a “PD stick.”
Single PD and dual PD
There are two different types of pupillary distance measurement; single PD and dual PD. The definition given earlier – the distance between the centre of your pupils – applies to single PD (also called binocular PD).
There is only one value in this measurement, hence the name. For example, a binocular PD could be written as 61mm.
Dual PD refers to the distance from the centre of each pupil to the middle of the nose, so there are two numerical values.
A dual PD of 31/30mm indicates that the centre of the right pupil is 31mm from the middle of the nose, and the centre of the left pupil is 30mm from the nose.
Monocular PD is another name for dual PD. The value of the right eye always comes first in monocular PD.
Either type of pupillary distance measurement will suffice when ordering glasses, but, it’s more common to use binocular PD. Dual PD is useful when ordering progressive lenses.
Reading glasses are an exceptional case, necessitating some simple subtraction. Take 3mm off your single PD or 1.5mm from each value of your dual PD to ensure your reading glasses meet your vision needs.
How to measure pupillary distance at home
Use the app
Our free pupillary distance app is the fastest and easiest way to measure PD. All you need is a smartphone, a computer with a webcam, and a magnetic strip card. The app has voice instructions that will guide you through each step of PD measurement.
Use a ruler
If you don’t have access to the tools required to use our app, you can easily measure your pupillary distance using this approach. All you need is a millimetre ruler and a mirror.
Step 1: Standing in front of a mirror, hold the ruler flat against the line of your eyebrows or up to the bridge of your nose.
Step 2: Close your right eye. While looking straight ahead into the mirror, align the ruler’s zero mark with the centre of your left pupil.
Step 3: Keeping the millimetre ruler still, close your left eye and open your right eye.
Step 4: Read the number aligned with your right pupil’s centre. This number is your first pupillary distance measurement.
Step 5: Repeat the process to ensure you get the most accurate measurement result for your left eye too.
If you are struggling to follow these steps, enlisting the help of a friend could make things easier. All you need to do is stand facing your friend and ask them to measure your PD using either a ruler or a millimetre tape measure.
Both of you can repeat the steps outlined above, with your friend holding the ruler to the bridge of your nose and taking the measurement. During this, your friend must make sure they keep their head still and look straight ahead while they read the measurement.
Is there an average pupillary distance?
Pupillary distance is different for each individual. The average PD for adults is between 54 and 74mm. If your own PD measurement falls outside of the average adult’s PD range, there is no need to worry.
High or low, the number itself does not have any effect on your vision. What’s important is that it accurately reflects the distance between your pupils.
The average pupillary distance for children is between 41 and 55mm. Since a person’s pupillary distance changes with physical development, it’s important to consider that children may have different pupillary distances each year. Don’t take for granted that their PD measurement from a previous year will still be correct.
If anyone knows how to measure pupillary distance, it’s your eye doctor. For the most precise reading possible, ask them to write it on your prescription card each time you or your child has an eye exam.
If you’re still unsure about any prescription-related issues, take a look at our helpful Optical Centre articles, or reach out to one of our certified opticians for help.