Pupillary Distance (PD) is a precise optical measurement that is more significant than many people realize when they order new eyewear. The PD is the distance, measured in millimeters, between the centers of your two pupils. This measurement determines the exact location of the optical center within each lens. Proper alignment ensures the corrective power is placed directly in front of your pupils, which is the precise point where light enters your eye. If the optical center is misaligned, the glasses will not function as intended, regardless of how accurate the prescription power might be.
Recognizing Symptoms of Misaligned PD
The most immediate sign that the PD in your new glasses may be incorrect is the persistence of uncomfortable visual symptoms. When the lenses are not centered properly, your eye muscles are forced to work harder to compensate for the misalignment, which quickly leads to strain. This often manifests as a dull, persistent ache in or around the eyes, particularly after periods of detailed visual work like reading or using a computer screen.
Many users report experiencing a centralized frontal headache, resulting from the continuous effort to fuse images from the off-center lenses. The visual field can feel distorted or “pulled,” making objects appear larger or smaller than they should. Some people experience mild double vision (diplopia), where images seem to separate slightly.
These symptoms are especially noticeable for individuals with higher prescription strengths or those wearing multifocal lenses, such as progressives, where the tolerance for error is small. While a minor adjustment period is normal for new eyewear, symptoms caused by an incorrect PD will not subside and may worsen over time. If visual complaints begin immediately and continue after a few days, the pupillary distance is a likely culprit.
How to Visually Inspect Your Glasses
While only a professional can verify the PD with specialized equipment, you can perform a simple visual check to estimate if the optical center is correctly placed. This point should align directly with the center of your pupil when you look straight ahead. A simple method involves standing in front of a mirror while wearing your glasses and using a fine-tipped, non-permanent marker.
Keeping your head still and looking straight ahead, have a friend carefully place a small dot on the outside of each lens directly over the center of your pupil. After marking both eyes, take the glasses off and measure the distance between the two dots in millimeters using a ruler. This measurement should closely match the PD number provided on your prescription or the number given to the manufacturer.
It is helpful to know whether your prescription uses a single PD or a dual PD measurement. A single PD is one total number representing the distance between the centers of both pupils. A dual PD (monocular PD) provides two separate numbers, measuring the distance from each pupil to the bridge of your nose. This dual measurement accounts for minor facial asymmetry, and the two numbers should add up to your total PD. If your self-measured distance is off by more than a millimeter or two from the prescribed PD, it suggests a misalignment.
Next Steps When PD is Incorrect
Once you suspect the Pupillary Distance is incorrect, stop wearing the glasses and immediately contact the dispensing optician or eye care professional. The issue requires professional correction. Explain the symptoms you are experiencing and mention that you suspect a PD error, which will prompt them to perform a verification.
The eye care professional will use a tool, such as a lensometer or a digital pupilometer, to precisely measure the placement of the optical centers in your lenses. This measurement confirms whether the actual PD placement deviates from the written prescription. Most industry standards allow for a small manufacturing tolerance, typically no more than 1 to 2 millimeters of deviation.
If the professional measurement confirms the PD is outside the acceptable tolerance range, the dispensing provider should take responsibility for the error. The lenses will typically need to be remade at no cost to you, as the error is considered a manufacturing or measurement mistake. Before a new order is placed, request that the PD measurement be taken again, preferably using a monocular (dual) method, to ensure the highest possible accuracy.