Pupillary Distance (PD) is the measurement between the centers of your two pupils, recorded in millimeters. This measurement is necessary for the correct manufacture of prescription eyewear. PD ensures that the optical center of each lens aligns precisely with the center of your pupils. Without this alignment, your eyes would be forced to work harder, potentially leading to visual discomfort.
While your eye doctor may measure your PD, it is often not included on the written prescription. This means you may need to obtain the measurement yourself when ordering glasses online. The measurement can be expressed as a single number (total distance) or as two numbers (distance from each pupil to the bridge of the nose).
The Difference Between Near and Distance Pupillary Distance
Near Pupillary Distance (Near PD) differs from Distance Pupillary Distance due to visual convergence. When focusing on a distant object, your lines of sight are nearly parallel, resulting in a wider distance between your pupils. When you look at something close, such as a book or screen, your eyes must turn inward to maintain focus on the near target.
This inward rotation, or convergence, physically reduces the distance between the pupils. Near PD is the measurement taken during this converged state and is required for lenses intended for close-up tasks, such as reading glasses, bifocals, or the near-vision segment of progressive lenses.
Your Near PD is 2 to 4 millimeters less than your Distance PD. This difference accounts for the convergence that occurs when you focus on a near object, usually considered to be about 16 inches (40 centimeters) away. Knowing your Distance PD allows you to calculate an estimated Near PD by subtracting about 3 millimeters from the total measurement.
Practical Step-by-Step Measurement Methods
Helper Measurement Method
Obtaining an accurate Near PD measurement requires simulating the natural convergence that occurs during reading. The most reliable technique involves using a friend or assistant. The user should sit and focus their gaze on the helper’s eye or nose while the helper stands at the intended reading distance, approximately 16 inches away.
The helper places a millimeter ruler horizontally across the user’s brow or nose bridge. They align the 0 mm mark with the center of one pupil and read the measurement at the center of the other pupil. The resulting number is the total binocular Near PD, which should be measured multiple times to ensure consistency.
Self-Measurement Using a Mirror
A second method allows for self-measurement using a mirror, though it can be challenging to maintain focus. Stand about 16 inches from a mirror and hold a millimeter ruler against your brow line. To encourage convergence, focus on an object placed just behind your reflection in the mirror at the same near distance.
Align the 0 mm mark with the center of your right pupil, then look straight ahead and note the reading that corresponds to the center of your left pupil. This process may be easier if you measure the monocular PD—the distance from the center of each pupil to the center of your nose bridge—and then add the two numbers together. A dual PD measurement, such as 32/30, is preferred as it accounts for slight facial asymmetries.
Verifying Accuracy and Using Your Measurement
An accurate Near PD is necessary because even a small error can compromise the effectiveness of your corrective lenses. When the optical center of the lens is misaligned, the light entering your eye is bent incorrectly, forcing your brain to work harder to reconcile the image. This added strain can lead to discomfort, including blurred vision, headaches, or eye fatigue.
To verify the accuracy of a self-measurement, repeat the process two or three times and calculate the average. The results should be consistent and fall within the expected adult range of 54 and 74 millimeters for a Distance PD. The Near PD should also be consistently smaller than the Distance PD if that measurement is available.
Once you have a reliable Near PD, provide it to the retailer or optician when ordering reading glasses or lenses for close-up work. An error of more than one millimeter can be significant, especially with higher prescription strengths or progressive lenses. Providing an accurate Near PD ensures the lens is manufactured to properly accommodate the natural inward movement of your eyes when you read.