Most people have never heard of pupillary distance before, but it's a very important index that you need to know when you are ordering eyewear. The pupillary may matter comfortable when you wear new glasses.
So, What Is Pupillary Distance?
Pupillary distance (PD) is the distance between two pupils, measured in millimeters. This sounds simple, but it's much more complicated than it sounds.
For starters, your pupillary distance will change, depending on whether you're focusing on an object that's close to your face or something far away. Think of what happens when you focus on an object and slowly bring it toward your nose: you look cross-eyed because your pupils are moving closer together. These two measurements are sometimes called near and far pupillary distance.
The Importance of Pupillary Distance
The pupillary distance can be different considerably from person to person. Men have an average pupillary distance of 64.0 mm, while women have an average of 61.7 mm, and distances between 54.0 mm and 68.0 mm are considered usual for both genders. Children, of course, will have pupillary distances that vary based on their age, sex and development.
When using PD to find a pair of fittable glasses, you may be tempted to just use the average pupillary distance for your gender. But it's essential to have your PD measured individually, and you should never guess or generalize.
If your pupillary distance is incorrect, you may notice that you don't see as clearly as you should. In some cases, problems with the pupillary distance of your eyewear may cause headaches, eyestrain or other discomforts that can make your glasses troublesome to wear.
Measuring Your Pupillary Distance
Because your pupillary distance isn't a set number, and it's measured in millimeters, it's very difficult to measure it at home without special equipment. Some websites offer rulers you can print out and use yourself while looking in a mirror, or you can have a friend do it.
However, measuring your pupillary distance is something usually left to professional optometrists. It's typically taken at the same time as other important measurements, like your vertex, wrap, and pantoscopic tilt, all of which affect the position of your eyeglasses.
Optometrists take pupillary distance measurements with a small millimeter ruler, called a "PD stick," or a machine called a corneal reflex pupillometer. This is a quick, painless measurement taken during a routine eye exam, often as one of the first statistics, your optometrist takes down.
Some software will also calculate your pupillary distance through a cell phone photo. But a lot can go wrong with a single photo, and thus a measurement taken this way can be quite inaccurate.
Single and Dual PD
In addition to near and far pupillary distance, your prescription may feature one of two statistics: monocular and binocular (also known as single and dual) PD. Monocular distance is the distance between the bridge of your nose and one of your pupils and is expressed as two measurements. Binocular distance is the direct distance between your two pupils.
Single PD may seem superfluous. After all, most people have a monocular distance that's the same for each eye, right?
In fact, because human faces aren't completely symmetrical, this isn't the case -- even a few millimeters' difference can cause vision problems. Plus, conditions like amblyopia ("lazy eye") can change just one pupil's pupillary distance far from the average.
Monocular and dual pupillary distance becomes very important for fitting certain kinds of glasses. Single vision lenses will usually take a monocular measurement, or just a near PD, while progressive lenses will always rely on a monocular measurement.
Bifocals and trifocals, on the other hand, will typically use a binocular near PD measurement. However, monocular PD is always more precise.
Reading Your Prescription
Your prescription might actually display your PD already. Just one number is your dual pupillary distance, but if you see two numbers, separated by a backslash, these are your two single measurements. The first is for your left eye, and the second is for your right.
Your pupillary distance may seem like a trivial thing, but it can have a huge impact on your eyewear. Some measurements may not even be useful for all prescription types, due to the potential difference between one eye's measurement and the other.
Thankfully, it's easy for an optometrist to measure your PD, quickly, accurately and easily. By getting the PD right when you order your lenses online, your glasses will be more comfortable to wear, and you'll avoid headaches and eyestrain.