What Is PD (Pupillary Distance) - How Is PD Measured
PD is one measurement you might notice written on your prescription. You’re probably wondering what that is, why and how much it matters. And as you are about to find out, this measurement is one of those small details that can make a huge difference in the quality of your lenses and how well you’re feeling when wearing your glasses.
In fact, the higher your prescription, the more important it is to get the right PD. Because if you don’t, even the slightest measurement errors of the PD can become a major inconvenience on a high prescription. Without further ado, here are the most common questions on PD, along with their answers:
What is PD and what does it stand for?
PD is the distance between the center of your nose and each eye. PD stands for “pupillary distance”. And judging by this simple definition, the PD is a value, expressed in millimeters, and you’ll see it as a number written on your prescription.
Because the human face is not perfectly symmetrical and you have two eyes, at the right and at the left of your nose, the distance from the center of your nose to the center of the right eye might be slightly different than the one to the center of your left eye.
Consequently, the PD is measured independently, for each eye. Your prescription should include an OD PD, which stands for the right eye PD, respectively an OS PD, which stands for the left eye PD.
How is PD measured in practice?
The most popular method to measure pupillary distance is the one involving a Pupilometer. This device is operated by an optician, who will place it in front of you. The device will automatically display the measurements for the right and left eye. This is the most accurate way to tell what PD you have.
Then, there is the alternative of isolating each eye at a time and using a permanent marker to mark its pupil center. This is, of course, a more complicated method that requires excellent skills from the person who will run the measurements. It also leaves considerably more room for errors, compared to simply using a Pupilometer.
Using a picture to measure the PD is not advisable. The reason is that this particular method will not account for convergence, which always leads to significant errors. And as you’ll see below, it is very important that you get the pupillary distance measured as accurately as possible.
How is PD written on the prescription, once measured?
Specialized laboratories operate with certain terminology. In some people, the left eye PD is equal with the right eye PD, which leads to a so-called binocular measurement.
When these values really are equal, the optician may choose to only write down a single value, one number that applies to both eyes.
Nevertheless, the rule of thumb is to use monocular measurements. This means to write down the numbers for each eye, even when they are identical.
What’s more, there is a Far Pupillary Distance and a Near Pupillary Distance for each eye, which totals four different measurements.
In a monocular measurement, you can see the numbers separated by a dash as in Left/Right values. In a binocular measurement, you can see the numbers separated by a dash as in Far/Near values.
Say your prescription displays for PD the values 54/51 or 28/27. You might interpret it as a representation of the Far/Near PD, or as a Left/Right PD.
One more thing you might want to know – even though there are four different PD measurements Far PD for the Right Eye, Far PD for the Left Eye, Near PD for the Right Eye, Near PD for the Left Eye, not all of them are absolutely necessary.
In fact, an eyeglass lens laboratory will only need to know the FAR PD value to produce the quality lenses that you need.
How important is PD and why?
Despite being described as a number, this value shows exactly where your eyeglass lens would have to have their optical center located. During the manufacturing process, the lenses will be modified as to be perfectly aligned with your eyes.
With the right measurement, the optical center of the lens will perfectly overlap the optical center of the eye. As you can tell, it’s an essential detail in the process of manufacturing top-quality.
What happens if the PD is not correctly measured?
Judging by the answer from the previous question, you can tell that if the PD is not correctly measured, wearing the glasses will put extra strain on the eye. The effect is known as prismatic imbalance, and it is translated through the continuous sensation that the objects you’re looking at are moving.
It’s also a pulling feeling, which leads to unnecessary discomfort, and that will make you want to avoid using the glasses. As already suggested above, this defect is even more obvious in higher prescriptions.
If the prescription is lower, the PD error may be less obvious or cause less eyestrain, but you’ll still feel that something is off about your glasses.
Could I measure my PD myself?
Chances are you’ll hear about many different ways that you can personally measure your pupillary distance, all by yourself. Such methods are described online and involve using rulers or credit card pictures, but none of them are actually accurate measurement options.
The error you’ll be getting from using these unofficial measurement options is way above the accepted errors withing the ANSI lens production.
Knowing how incorrect PD measurement can cause you eyestrain and even contribute to deteriorate your vision, you’ll probably want to think twice about measuring the PD yourself.