When ordering new eyewear, users often encounter a request for their Pupillary Distance (PD), which can be confusing, especially when two different numbers are presented. This frustration arises because the correct number depends entirely on the specific purpose of the lenses being purchased. Understanding the difference between distance and near PD measurements is paramount to receiving glasses that provide clear vision and comfortable wear. This article details the optical principles behind these two numbers and provides clear instructions for selecting the correct measurement for your prescription.
Understanding Pupillary Distance
Pupillary distance (PD) is the measurement, recorded in millimeters (mm), between the centers of your two pupils. This number is the foundation of accurate lens grinding and mounting within the frame, as it dictates the precise horizontal placement of the lens’s optical center.
The optical center is the single point on the lens where light passes through without being bent or distorted by the curvature of the lens. If the optical center is not aligned precisely with the center of your pupil, you will experience a prismatic effect. Misalignment forces the eye muscles to constantly work to pull the image back into focus, leading to symptoms like eye strain, headaches, and visual distortion. Accurate PD ensures that the lens delivers the correction exactly where the eye needs it.
The Optical Reason for Two Measurements
The reason two PD measurements exist is rooted in the natural movement of the eyes when shifting focus. When you look at an object located at a distance, your eyes are looking straight ahead and are generally parallel. The measurement taken in this relaxed state is called the Distance PD.
When focusing on a nearby object, such as a book, your eyes must naturally turn inward to maintain single, clear vision. This coordinated inward turning is known as convergence. Because the eyes are converged, the physical distance between the centers of the pupils becomes slightly smaller.
The resulting measurement when the eyes are converged for reading is the Near PD. Typically, the Near PD is between 2 to 4 millimeters less than the Distance PD. This small difference accounts for the eye’s anatomical change when performing sustained close-up tasks.
Selecting the Correct PD for Your Lens Type
The type of lens ordered directly determines whether the distance or near measurement is appropriate. For single vision lenses intended for seeing objects far away, such as when driving, the Distance PD must be used. These lenses are designed for use when the eyes are in their naturally parallel position, requiring the full, wider measurement to center the lens correctly.
If ordering single vision reading glasses or computer glasses, the Near PD is the correct measurement. Since these lenses are designed exclusively for close-range tasks where the eyes are converged, using the Distance PD would incorrectly place the optical center too far apart, causing strain. If a Near PD is not provided on your prescription, subtract 2mm or 3mm from the Distance PD to approximate the converged measurement.
Progressive addition lenses offer multiple focal lengths within a single lens and require a high degree of precision in their fitting. These lenses contain a distance viewing area at the top that transitions gradually to a near reading area at the bottom. For online ordering, retailers may ask for both the Distance PD and a specific Near PD, sometimes requiring a dual PD measurement.
Finding or Measuring Your Pupillary Distance
A challenge when ordering glasses online is that the PD measurement is often omitted from the standard eye prescription. The most accurate method for obtaining this value is to contact the prescribing doctor or clinic and request the measurement be released to you. Since the PD is a physical measurement that rarely changes in adulthood, this number can be reliably used for many years.
If contacting the eye doctor is not feasible, the measurement can be taken at home using a ruler and a mirror or a friend. When self-measuring, look straight ahead into the distance. Align the zero mark of a millimeter ruler with the center of one pupil, then read the measurement at the center of the other pupil. This method yields the Distance PD, which can then be adjusted for near-vision glasses.
For maximum accuracy, especially with stronger prescriptions or progressive lenses, the optometrist usually provides a Dual PD. This consists of two numbers (e.g., 31mm/32mm), measuring the distance from the nose bridge to the center of each individual pupil. Using the dual measurement accounts for slight facial asymmetries and ensures the optical center is perfectly aligned for each eye independently.