The Pupillary Distance (PD) value is a fundamental measurement used when ordering prescription eyewear. It represents the precise distance between the centers of your pupils, measured in millimeters (mm). This value is necessary to ensure that the corrective portion of your eyeglass lenses aligns perfectly with your eyes’ visual axis. Without an accurate PD, the lenses cannot be positioned correctly, which compromises the quality of vision correction. Obtaining this measurement is necessary for anyone purchasing new glasses, especially when ordering online.
Defining Pupillary Distance
Pupillary distance (PD) is the physical measurement between the centers of the pupils in both eyes. This measurement is expressed as a single number in millimeters, typically ranging from 54mm to 74mm for adults. The PD is a static measure that determines where the optical center of each lens must be placed.
The PD value is important because it dictates the placement of the lens’s optical center. This is the specific point on the lens where light passes through without being bent, ensuring the clearest part of the correction aligns with the wearer’s line of sight. For optimal vision and comfort, the optical center must line up precisely with the center of the pupil.
If the PD used to manufacture the glasses is incorrect, the wearer looks through a section of the lens that is not the intended optical center. This misalignment forces the eye muscles to work harder to compensate for the deviation, which introduces visual distortion. An accurate PD ensures the corrective power is delivered effectively and comfortably.
Different Pupillary Distance Measurements
PD measurements are classified into two types: binocular and monocular. The binocular PD is a single measurement representing the total distance between the center of the right pupil and the center of the left pupil.
The monocular PD, also called dual PD, involves two separate measurements. This measurement is taken from the center of the nose bridge to the center of each individual pupil, resulting in two distinct numbers (e.g., 32mm/31mm). The two monocular measurements always add up to the total binocular PD.
Monocular PD offers greater precision because it accounts for facial asymmetry. The distance from the nose bridge to the right pupil may differ slightly from the distance to the left pupil. While binocular PD is adequate for simple, low-power, single-vision lenses, monocular PD is recommended for complex prescriptions, such as progressive lenses or higher-powered corrections.
Obtaining and Verifying Your PD
The most reliable way to obtain your PD is through a professional measurement conducted by an eye care specialist. Opticians use a specialized instrument called a pupillometer, which provides the most accurate reading. This measurement is often recorded on your prescription slip, but sometimes it must be requested separately.
For convenience, especially when ordering glasses online, many people measure their PD at home. A common method uses a standard millimeter ruler and a mirror. To perform this measurement, stand 8 to 18 inches away from a mirror and hold the ruler horizontally against your brow line.
To measure, close your left eye and align the ruler’s zero mark exactly with the center of your right pupil. While keeping the ruler still, open your left eye, close your right eye, and read the millimetre mark that aligns with the center of the left pupil. This resulting number is your binocular PD. Repeating the process two or three times helps ensure a more consistent result.
The Consequences of Inaccurate PD
Wearing glasses made with an incorrect PD leads to uncomfortable symptoms, including eye strain, persistent headaches, and general visual discomfort. When the optical center is misaligned with the pupil, the eye is forced to constantly work harder to compensate, leading to fatigue.
This misalignment introduces an unwanted prismatic effect. Since a prism shifts the image seen by the eye, unintentional shifting forces the eyes to constantly converge or diverge to fuse the image, causing stress. This visual distortion can also lead to blurry vision, poor depth perception, and objects appearing to be the wrong size or shape.
The severity of these symptoms relates directly to the degree of the PD error and the strength of the prescription. For high-powered or complex prescriptions, an error of one or two millimeters can create significant unwanted prism and visual stress. While an incorrect PD does not cause permanent damage to adult eyes, it severely reduces the effectiveness of the prescription and makes the eyewear uncomfortable.