Pupillary Distance (PD) is the measurement of the space between the centers of your two pupils, typically recorded in millimeters. This measurement is necessary for the proper fabrication of prescription eyewear, as it dictates where the optical center of each lens must be placed. Providing an accurate PD is required, especially when ordering glasses online, to ensure the lens centers align with your line of sight. Correct alignment guarantees you look through the clearest part of the lens, providing the best visual correction. If the optical center is misaligned, the glasses will not function as intended and may cause discomfort, even if the rest of your prescription is correct.
Understanding Pupillary Distance
Pupillary Distance is measured in two ways: binocular and monocular. Binocular PD is a single number representing the total distance between the centers of the two pupils. This is the most common measurement and is often sufficient for standard, single-vision lenses. The typical adult binocular PD ranges between 54 and 74 millimeters, averaging around 63 millimeters.
Monocular PD, also called dual PD, involves two separate measurements: the distance from the center of the nose bridge to the center of the right pupil and the distance to the center of the left pupil. These two numbers combine to form the total binocular PD. Monocular measurement is important because most people have slight facial asymmetry, meaning the distance from the nose bridge to each eye is not identical. This separate measurement is often required for higher-strength or complex lenses, such as progressives, where precise alignment for each eye is necessary for optimal vision.
Step-by-Step Guide for Self-Measurement
The most straightforward method for finding your binocular PD involves using a millimeter ruler and a friend. Measuring with a friend is generally more accurate than measuring alone because it allows you to look into the distance, which is necessary for a distance PD measurement. You should focus on an object about 10 to 20 feet away, keeping your eyes still and looking straight ahead.
Your friend should hold the ruler horizontally across the bridge of your nose, ensuring the zero mark aligns exactly with the center of one pupil. While you look straight ahead, they must read the millimeter mark that lines up precisely with the center of your other pupil. This single number represents your distance binocular PD. Repeat this process at least two to three times and use the average of the consistent numbers to increase confidence in the result.
Monocular Measurement
A different approach is needed for monocular measurement, which is taken from the pupil to the center of the nose bridge. For this, your friend aligns the ruler’s zero mark at the center of the bridge of your nose. They then record the measurement over the center of your right pupil, and separately, the measurement over the center of your left pupil. This results in two numbers, such as 32/31, which are used for lenses requiring individualized placement.
Ensuring Accuracy and Avoiding Common Mistakes
Precision in Pupillary Distance measurement is important, as even a small error can affect visual comfort. A tolerance of plus or minus 1 to 2 millimeters is generally acceptable for lower prescriptions, but the margin for error decreases significantly with stronger prescriptions. An incorrect PD causes the wearer to look through a part of the lens that is not the optical center, which induces an unwanted prismatic effect.
This induced prism forces the eyes to strain to compensate for the light bending incorrectly. The result of this misalignment can be symptoms like blurred vision, eye strain, persistent headaches, or distorted depth perception.
Common Errors
A common mistake during self-measurement is the parallax error, which occurs when the ruler is not held directly in front of the eyes or when the measurer looks at the ruler from an angle. Another frequent error is measuring for distance PD while focusing on the ruler, which causes the eyes to converge and results in a narrower, near PD number.
Alternatives to DIY Measurement
If you are concerned about the accuracy of self-measurement, professional options are available. The most accurate way to obtain your PD is to have it measured by an optometrist or an optical shop. They typically use a specialized device called a pupillometer, which provides a highly precise measurement. Although PD is necessary for making glasses, it is often not included on your written prescription, so you may need to specifically request it.
Many online eyewear retailers also provide digital tools and mobile applications that can measure your PD using a smartphone camera. These apps typically require you to take a photo or video while holding a standard-sized object, like a credit card, next to your face for scale. Some newer smartphone models use depth-sensing technology to achieve an accuracy of up to 0.5 millimeters. Another option is having the PD measured from an existing pair of glasses by an optical professional, provided those glasses fit correctly.