Pupillary distance (PD) is a foundational measurement required for manufacturing prescription eyeglasses. This distance ensures the optical center of the lens is perfectly aligned with the center of the wearer’s pupils. Correct alignment allows light to pass through the most precise part of the lens, maximizing vision correction. Without this measurement, lenses cannot be centered properly, which introduces visual distortions. Measured in millimeters, PD is as important as the prescription itself for producing comfortable and clear vision.
What is Pupillary Distance (PD)?
Pupillary Distance is the physical measurement of the space between the centers of the pupils. The goal is to place the optical center of the lens—the point of clearest vision correction—directly in front of the pupil. The PD can be measured in two primary ways: binocular or monocular PD. Binocular PD is a single number representing the total distance between the pupils. Monocular, or dual PD, provides two separate measurements from the nose bridge to the center of each individual pupil, which is considered more precise since most faces are not perfectly symmetrical.
Far PD: The Standard Distance Measurement
Far PD, or Distance PD, is the measurement taken when the eyes focus on an object 20 feet (about 6 meters) or more away. At this distance, the eye muscles are relaxed, and the lines of sight are nearly parallel, representing the widest pupillary distance. Far PD is the standard measurement used for everyday glasses. It is required for all single-vision distance glasses, such as those worn for driving or walking. It also forms the basis for the distance-viewing segment in multifocal lenses, including the upper portion of progressive and bifocal lenses.
Near PD: The Reading and Close-Up Measurement
Near PD, or Reading PD, is the measurement taken when the eyes are focused on an object held close to the face, typically at a reading distance of about 16 inches (40 centimeters). When focusing on a close object, the eyes naturally turn inward toward the nose in a process called convergence. This muscular action effectively reduces the distance between the pupils, meaning Near PD is always a smaller number than Far PD. The difference between the two measurements is usually a reduction of 2 to 4 millimeters. This measurement is necessary for manufacturing dedicated reading glasses, computer glasses, and the near-vision segment located in the lower portion of progressive lenses.
Why Accurate PD is Critical for Lens Performance
The PD measurement dictates the precise location of the lens’s optical center, where light passes through without being bent. If the PD used to manufacture the glasses is incorrect, the wearer’s line of sight will pass through a point away from this center. This misalignment introduces an undesirable visual effect known as induced prism. Induced prism means the lens incorrectly bends the light, forcing the eye muscles to strain and compensate for the error, leading to symptoms like eye fatigue, blurred vision, headaches, and nausea. The need for accuracy is amplified significantly with higher-power prescriptions, where minor decentration can cause a substantial prismatic effect.