What Is Pupillary Distance (PD) Means In Glasses?

The pupillary distance is used to center your eyeglass prescription. Your eyeglass provider should measure this for you, but it won’t appear on your prescription, so you will need to find this measurement for yourself if you order glasses online.

The Pupillary Distance: What Is It?

The pupil distance, also known as the interpupillary distance, is a measurement of the distance between your pupils. A pupil is the dark center of your eye that widens to receive light or narrows to focus on a close object. They aid in your vision in the dark or in bright conditions. When you purchase glasses, you will need to know your pupil distance, one of several measurements you need to make.

Single vs. Dual Pupillary Distance

A pupillary distance is composed of two types, and they can vary from one location to another.

Far pupillary distance (FPD). Is the distance between the centers of one pupil and the other when you are looking at something off in the distance. Your eye care provider will normally check this PD unless you are buying computer or reading glasses.

The near pupillary distance. Measures how far your eyes are from the center of your pupils when you are focusing on something close to your face, and it is usually about 3 to 4 millimeters shorter than the far pupillary distance.

Additionally, there are two types of measurements.

Single pupillary distance. Known also as binocular pupillary distance, this measurement is the total distance between the centers of both pupils. It is called single because it only has one number or binocular because both eyes are included. It is the most common method of measurement.

Dual pupillary distance. Also known as monocular pupillary distance, this is the distance between the center of one pupil and the bridge of your nose. A dual pupillary distance has two numbers, one for each eye. It is common for one side to differ slightly from the other.

What Is Pupillary Distance Used For?

You may use pupillary distance to size your glasses, but it’s not usually important in managing your vision treatment. Your eye doctor might use it to check parts of your vision or eye function. Therefore, it is not included in your prescription, even though you need it to order glasses.

When fitting glasses, you should ensure the center of your lens sits directly in front of your pupil for the best viewing experience. In order to reduce eye strain, it is best to align the center of your prescription with your pupil. However, depending on the type of glasses you purchase, the measurement will vary.

The optician may use a handheld instrument called a pupillometer or a digital PD meter to measure your pupillogram. These devices are held up to your face and adjusted so that the correct distance is achieved. A special ruler might also be used to measure, or a pair of glasses might be marked and then the distance between each pair of lenses measured.

Test for stereo acuity. In order to understand your stereo acuity, your eye doctor will use PD. While both eyes see images and objects individually, your brain puts those left and right visions together to create a three-dimensional image. Your doctor can use pupillary distance to determine your left and right eyes’ differences in vision.

Testing for near-point convergence. This happens when the eyes are focused on an object that is close to your nose and your doctor will observe how well your eyes work together to focus on the object.

When your eyes converge, or come together, your pupillary distance decreases, so your physician may check to ensure that you are experiencing PD changes.

When fitting reading glasses, it is particularly important to take into account the pupillary distance.

What Is the Average Pupillary Distance?

In adults, the average pupillary distance is 63 millimeters, although most adults have PDs between 50 and 75 millimeters. In children, the average pupillary distance is at least 40 millimeters.

There is, however, a need to measure your own pupillary distance and not rely on averages. Each individual has a different pupillary distance, which differs based on age, gender, and ethnicity. It also fluctuates a great deal with age, up to the age of 30. It is important to remeasure a child when buying new glasses because the child’s pupillary distance changes the most between birth and age 19.

How to Measure Pupillary Distance

In order to order glasses online, you will need to measure your pupillary distance. Some companies offer tools that can assist with this procedure, but it is simple enough to perform on your own.

PD can be measured by yourself in the following way:

1. Stand 8 inches away from the mirror.

2. Place a ruler in front of your eyebrow and ensure that you are measuring in millimeters.

3. Close your right eye and place the 0-millimeter mark in the center of your left pupil.

4. Look straight ahead with your right eye, then close your left eye.

5. Using your left eye, find the line that intersects the center of your pupil. This is your pupillary distance.

To measure with a friend, follow the instructions below. If you have trouble closing one eye, or you find it difficult to measure by yourself, ask a friend or family member for assistance.

1. Look straight ahead as you stand about 8 inches away from your friend.

2. They should place the millimeter ruler on the bridge of your nose.

3. Your friend should line up the 0 millimeters with the center of your left pupil by covering your right eye with your hand.

4. Uncover your eye, look straight ahead, and then cover your left eye.

5. A friend will align the ruler with the center of your right eye. Your pupillary distance will be determined as a result.

Calculate the average of the three measurements you made and repeat the procedure three times. This will allow you to obtain a more accurate distance measurement.

What Happens If Your Pupillary Distance Is Off?

Having the correct pupillary distance determines how comfortable and clear your glasses prescription will be. If you have the wrong measurement, you may experience blurriness, headaches, or eye strain.

In either case, you can make mistakes whether you measure your pupillary distance manually or your optician does it with a PD meter. This usually occurs as a result of the meter being too low on your bridge or not looking straight ahead when you measure your pupillary distance. Even so, it is best to have your PD measured by a professional, especially if you wear progressive lenses.

You can ask your eyeglass provider or eye doctor’s office to measure your children’s glasses for you if you are having trouble getting the measurement or you are concerned that it is incorrect. Some providers might perform the service for free, while others might charge you a fee.