Pupillary Distance (PD) is the measurement, in millimeters, between the centers of your two pupils. PD ensures that the corrective part of the lens is placed directly in front of your eye’s center, which is the point you look through. Understanding the importance of this measurement, especially when purchasing glasses online, is the difference between clear, comfortable vision and unusable lenses.
The Optical Necessity of Pupillary Distance
The primary function of a corrective eyeglass lens is to refract, or bend, light precisely onto the retina at the back of the eye. Every prescription lens has a specific point called the optical center, where light passes through without being bent at all. For the glasses to work as intended, this optical center must align perfectly with the center of your pupil.
The Pupillary Distance measurement provides the lab with the exact horizontal placement for this optical center within the frame. If the lenses are manufactured with a PD that is too wide or too narrow, the light passing through the lens will be bent incorrectly. This misalignment forces your eye muscles to strain as they attempt to compensate for the error.
The higher the power of a patient’s prescription, the more sensitive the lenses become to even small errors in PD. A lens with a strong prescription has a greater light-bending ability, meaning that moving just a millimeter away from the optical center results in a much larger visual distortion than it would for a weaker lens. A precise PD is more important for people with higher degrees of nearsightedness or farsightedness.
Obtaining and Using Your PD Measurement
While your eye doctor determines your prescription, the PD is a separate measurement that may not always be included on the written prescription. This is because PD is traditionally a dispensing measurement taken by an optician. The average adult PD typically falls between 54 and 74 millimeters, but relying on an average is not recommended, as even a small deviation can cause problems.
Professional measurements are usually taken by an eye care specialist using a specialized ruler called a pupillometer or a digital measuring device. Consumers can also attempt to measure their own PD using a ruler and a mirror, or by utilizing smartphone applications, but the accuracy of these self-measurements can vary significantly.
PD can be expressed in two main ways: Single PD (also called binocular PD) or Dual PD (also called monocular PD). Single PD is one number representing the total distance between the center of both pupils. Dual PD is two numbers, measuring the distance from the center of the bridge of the nose to the center of each pupil individually (e.g., 32/31).
Dual PD is the preferred and more accurate method because it accounts for the fact that a person’s eyes are rarely perfectly symmetrical in their distance from the nose. This is relevant when ordering progressive or multifocal lenses, which require precise placement of different corrective zones. Also, a Near PD measurement is sometimes required for reading-only glasses, as the eyes naturally converge, or turn inward, when focusing on a close object.
The Visual Consequences of an Incorrect PD
When glasses are manufactured using an incorrect Pupillary Distance, the resulting misalignment of the optical centers can lead to visual symptoms. This error forces the eyes to work hard to merge two slightly misaligned images into a single, clear picture. The most common physical symptoms are headaches, eye strain, and general eye fatigue, which often worsen while wearing the glasses.
The scientific reason behind these symptoms is the creation of induced prism, which is an unintended bending of light caused by looking through the lens away from the optical center. This unwanted prismatic effect displaces the image, requiring the eye muscles to constantly adjust to bring the image back to the center of focus. If the PD is off horizontally, the eyes are forced to either converge or diverge more than is natural, leading to blurriness, distortion, and sometimes double vision.
Industry standards, such as those set by the American National Standards Institute (ANSI), define the maximum acceptable tolerance for PD errors, measured by the amount of unwanted prism induced. For most prescriptions, a horizontal PD error of more than a few millimeters can exceed these standards and cause noticeable discomfort. Wearing glasses with an incorrect PD does not typically cause permanent damage to the eyes, but the continuous strain can lead to chronic headaches and tired eyes.