Pupillary Distance (PD) is the precise measurement of the distance between the centers of your two pupils, typically expressed in millimeters. This measurement is required when purchasing new prescription eyeglasses because it dictates where the optical center of your lenses should be placed in the frame. If the lenses are not aligned correctly with your pupils, you may experience visual discomfort, eye strain, or headaches, even if the prescription power is accurate. An accurate PD ensures that the light rays are focused correctly through the lens and into your eye, optimizing your vision correction.
Preparing for Measurement
Before beginning the measurement process, gather a few simple materials to ensure an accurate result. The most important tool is a ruler marked with millimeter (mm) increments, as pupillary distance is a precise measurement. You will also need a small mirror or a reliable friend to assist you, and good lighting to clearly see your pupils and the ruler markings.
A ruler designed specifically for PD measurement can be helpful, but any standard ruler with millimeter markings will work. The measurement should be taken to the nearest half-millimeter for the best results.
Step-by-Step PD Measurement Guide
The most common measurement, known as binocular PD, determines the single distance between the center of the right pupil and the center of the left pupil. Stand approximately eight to twelve inches away from a mirror, or have your helper stand directly in front of you. Keep your head perfectly straight and look directly ahead without moving your eyes.
Hold the millimeter ruler horizontally across the bridge of your nose, ensuring it is level and stable. The ruler should be positioned as close to your eyes as possible without touching them. If using a mirror, close your right eye and align the zero mark of the ruler precisely with the center of your left pupil.
While keeping the ruler steady and the zero aligned, close your left eye and open your right eye. Without moving your head or the ruler, read the measurement that lines up exactly with the center of your right pupil. This number, typically between 54 mm and 74 mm for adults, represents your binocular PD.
Closing one eye and then switching counteracts the slight shift in focus that occurs when measuring yourself in a mirror. This simulates the alignment needed for distance vision glasses. Repeat the entire process two or three times to verify that you consistently arrive at the same measurement.
Understanding Monocular PD and Ensuring Accuracy
While binocular PD provides a single number for the distance between both pupils, monocular PD gives two separate measurements: one for the right eye and one for the left eye. This dual measurement is the distance from the center of the nose bridge to the center of each pupil individually. Monocular PD is often preferred for higher-power prescriptions, progressive lenses, or in cases of facial asymmetry, as it allows for more precise optical centering for each lens.
To measure monocular PD, adapt the previous steps to measure each eye separately from the center of your nose. First, align the zero mark of the ruler with the exact center of your nose bridge. Then, look straight ahead and read the measurement that aligns with the center of your right pupil.
Next, without moving the zero mark from the nose bridge, read the measurement that aligns with the center of your left pupil. These two numbers, which should add up to your total binocular PD, are recorded as the right eye (OD) and left eye (OS) monocular measurements. It is common for these two numbers to differ by a millimeter or two due to natural variations in facial structure.
A common error in self-measurement is inadvertently looking away from the mirror or failing to keep the ruler perfectly level. Any slight tilt of the head or the ruler can introduce an error of one or two millimeters, which can affect lens performance. Always take multiple measurements to ensure the highest degree of accuracy for your new eyewear.