Pupillary distance (PD) measures the distance between the centers of your pupils, typically in millimeters. This measurement is important for ensuring that the optical center of your eyeglass lenses aligns correctly with your pupils. Accurate alignment is crucial for clear, comfortable vision, ensuring the lenses are positioned accurately within your chosen frames.
The Role of Pupillary Distance in Vision
PD plays a role in how effectively your eyeglasses correct vision. When lens optical centers do not align with your pupils, it can lead to visual discomforts like blurred vision, eye strain, and headaches. This misalignment can also cause a “prismatic effect,” where light is bent incorrectly as it passes through the lenses, distorting vision.
An accurate PD measurement ensures you look through the most precise part of your lenses, minimizing distortion and maximizing clarity. Without correct PD, your eyes may work harder to focus, leading to fatigue and discomfort, especially when wearing glasses for extended periods.
Professional PD Measurement Methods
Eye care professionals use several methods to accurately measure pupillary distance. These include electronic pupillometers, manual ruler measurements, and auto-refractors used during eye exams.
Professionals measure different types of PD for specific visual needs. Distance PD measures the distance between pupils when focusing on distant objects, typically used for general vision correction. Near PD measures the distance when focusing on close objects, important for reading or computer glasses. Monocular PD measures the distance from the center of each pupil to the bridge of the nose separately, beneficial for complex prescriptions like progressive lenses or if there is facial asymmetry.
Measuring Your Own Pupillary Distance
Measuring your own PD at home is possible with a few simple tools, though professional measurement is generally recommended for the highest accuracy. You will need a metric ruler, a mirror, and optionally a pen or a helper.
For distance PD, stand about 8 inches (20 cm) from a mirror. Hold the ruler horizontally against your brow, ensuring the millimeter markings are visible. Close your right eye and align the ruler’s 0 mm mark with the center of your left pupil. Without moving the ruler, open your right eye and close your left eye. The millimeter mark aligning with your right pupil’s center is your PD. Repeat this process two to three times for consistency and accuracy.
If using a helper, focus on an object 10 to 20 feet away. Have your helper hold the ruler across your brow, aligning the 0 mm mark with one pupil’s center, then measure to the other. For near PD, subtract approximately 3-4 mm from your distance PD measurement.
Understanding Your PD Readings
Understanding your PD readings helps in selecting appropriate eyewear. Binocular PD is a single number representing the total distance between both pupils (e.g., 60mm), commonly used for standard distance vision glasses. Monocular PD (Dual PD) provides two separate numbers, one for each eye, indicating the distance from each pupil’s center to the bridge of your nose (e.g., 30/30mm or 29/31mm). This is preferred for more precise lens placement, especially with progressive lenses.
Distance PD is typically larger than near PD because your eyes naturally converge when focusing on closer objects. Near PD is usually 2-4 mm shorter than distance PD. The average pupillary distance for adults generally ranges between 50 and 75 millimeters, with an approximate average of 63 millimeters. Children typically have a smaller average PD, and their PD can change as they grow.