Pupillary Distance (PD) is a necessary measurement that determines how prescription lenses are manufactured and fitted into eyeglass frames. Measured in millimeters, PD tracks the span between the center points of your pupils. The accuracy of this number ensures that your eyes look through the exact center of the corrective lens, where the full, undistorted power of your prescription is delivered.
Defining Distance Pupillary Distance (PD)
Distance Pupillary Distance (Distance PD) is the measurement taken when the eyes are focused on an object 20 feet or more away. At this distance, the eyes are in a relaxed, parallel state, which is necessary for making lenses used for driving or general wear.
The resulting value is expressed in millimeters (mm) and is fundamental for the correct placement of lenses within the frame. For most adults, this measurement falls between 54 mm and 74 mm. While the average adult PD is often cited around 60 to 64 mm, the specific number is unique to each individual.
Why PD is Important for Lens Accuracy
The primary function of the PD measurement is to determine the location of the Optical Center (OC) in the lenses. The OC is the single point where light passes through without being bent or distorted by the lens curvature. For optimal vision and comfort, this Optical Center must align perfectly with the center of your pupil when you look straight ahead.
If the manufactured lens is placed incorrectly, even by a small amount, the eye is forced to look through a non-optimal part of the lens. This misalignment introduces an unwanted optical effect known as induced prism, which shifts the image away from its true location. The eye muscles must work harder to compensate for this shift, which leads to uncomfortable symptoms.
Wearing glasses with an inaccurate PD can result in visual discomfort, including persistent headaches, generalized eye strain, and blurred vision. The higher your lens prescription power, the more sensitive your vision becomes to small errors in PD alignment. Difficulties with depth perception and adaptation challenges are also consequences of a poorly centered lens.
Understanding the Measurement Formats
Pupillary Distance can be recorded on a prescription in a couple of different formats. A Single PD, also known as binocular PD, is represented by one number, such as 62 mm, which is the total distance between the two pupil centers. This single number is commonly used for standard single-vision lenses where the prescription power is uniform across the lens.
The other common format is Dual PD, or monocular PD, which provides two separate numbers, such as 31/31 or 32/30. These numbers represent the distance from the center of the nose bridge to the center of each pupil individually. Dual PD is considered a more precise measurement because it accounts for the fact that most people have some degree of facial asymmetry.
Dual PD is particularly useful for complex lens designs like progressive or bifocal lenses, where different focal zones must be precisely aligned for a smooth transition. Another distinction is between Distance PD and Near PD. Near PD is measured when the eyes converge, or turn slightly inward, to focus on an object close up, such as a book. Because the eyes are turned inward, the Near PD is always a few millimeters less than the Distance PD.
Prescriptions for general-purpose glasses typically only list the Distance PD. However, a Near PD measurement is necessary if the glasses are intended only for reading or if the lenses are progressive or bifocal.