If you have ever purchased eyeglasses you have probably heard of the measurement of the eyes called the pupillary distance (PD). If you’ve ever ordered glasses online, you are probably even more familiar with this “PD measurement”, and may have even attempted to measure it yourself at some point. Why do you need this number to order glasses? Truth is, without this number, your glasses will not fit you correctly and you will likely experience blurry vision and headaches.
The pupils are in the center of our eyes. They appear as black circles in the center of the colored part of our eyes, called the iris, and it is actually a muscle that opens (dilates) and closes (constricts) to let more or less light into our eyes. At night, our pupils dilate to let more light in so we can see well.
If you think about the structure of the eye, the pupils are in the center to direct light onto the center of the retina. You can think of the focusing of what you see like a target with a bull’s-eye in the middle. The images we see focus through the pupil center onto this bulls-eye.
When you get glasses made, the pupil distance measurement is taken to ensure that the lens center is in line with your pupil center. This measurement is absolutely necessary to achieve precise vision. If your pupil distance does not match where the centers of your pupils are, your vision can be affected– Like wearing someone else’s glasses!
The wrong PD can induce eye strain, fatigue, headaches and blurry vision. If you have a high prescription and the wrong PD these symptoms are often much worse. In my practice, some patients will complain that they “just don’t feel right”. Sometimes it is a vague sense that something is wrong with their glasses.
Different Types of PDs
There are two types of PD’s that are measured. One is the binocular PD where we measure between the two pupils. The other is the monocular PD which is measured from the center of one pupil to the center of the bridge of your nose. Monocular PD is more accurate because your eyes may not be equally centered, and the monocular PD can adjust for this imbalance. Monocular PD’s are used for progressive lenses and bifocals to ensure that the centers are as perfect as possible. Binocular PD’s can be used for people with lower prescriptions as it usually will not induce the eye strain or other symptoms.
How To Get Your PD
- Visit your Optometrist. They use a device called the “pupillometer” to accurately measure pupillary distance.
- Do It Yourself, using a PD ruler. EyeQue offers a free PD ruler with every Personal Vision Tracker purchase. If you need one, let us know at firstname.lastname@example.org. We even have a tutorial, check it out here.
- Do It Yourself, using a printable ruler.
- Do It Yourself with a friend, using an actual ruler. It’s a bit harder and will require some calculations.