The landscape of prescription eyewear has rapidly changed, with many consumers turning to online retailers for convenience and lower costs. This shift requires individuals to provide their prescription and a measurement traditionally handled only by an optician. This measurement, known as Pupillary Distance (PD), determines the precise alignment of the lenses within the chosen frames. Understanding PD is now a standard requirement for anyone purchasing corrective lenses outside of a professional’s office.
Understanding Pupillary Distance
Pupillary distance (PD) is the measurement, recorded in millimeters, of the space between the centers of your two pupils. Sometimes called interpupillary distance, this metric is fundamental to creating prescription glasses. The average PD for adults ranges between 54 mm and 74 mm, though individual measurements can vary.
The primary function of this measurement is to ensure the optical center of the lenses is correctly positioned directly in front of the center of your pupils. The optical center is the specific point on the lens that provides the clearest vision and the full benefit of the prescription. For those using multifocal lenses, such as progressives or bifocals, a near PD measurement, which accounts for the slight narrowing of the pupils when focusing up close, is also often required.
The Necessity of Accurate Measurement
You definitively need your pupillary distance to order any prescription eyewear, especially when ordering online. This measurement guides the lens manufacturer in placing the optical center of your corrective lenses. If this center is misaligned, light rays entering your eye will be bent incorrectly, a phenomenon known as induced prism.
This unwanted prismatic effect forces the eye muscles to work harder to converge or diverge to compensate for the deviation. Even a small error of just one or two millimeters can create noticeable visual discomfort, particularly for individuals with higher lens powers. The precision of the PD measurement directly impacts the comfort and effectiveness of the final product. The higher the prescription strength, the more pronounced and disruptive an incorrect PD measurement becomes.
Obtaining Your Pupillary Distance
There are three primary ways to obtain the PD measurement. The most accurate method involves requesting the measurement from your eye care professional, optometrist or optician, who often uses a specialized device called a pupillometer. While some regulations require this information to be included with your prescription, you may still need to specifically ask for the PD number.
Many online glasses retailers now offer digital tools, often utilizing a webcam or a photo with a reference object like a credit card, to calculate the PD. These tools provide a convenient and generally reliable option for obtaining the measurement at home.
Self-measurement is a third option, using a millimeter ruler and a mirror or with the help of a friend. If using a friend, look at a distant object (10 to 20 feet away) to ensure correct eye positioning. For the highest accuracy, especially with complex prescriptions, use the dual or monocular PD, which measures the distance from the center of each pupil to the bridge of the nose separately.
Effects of an Incorrect Measurement
Wearing glasses with an incorrectly calculated PD can lead to uncomfortable symptoms. These effects result from the eyes struggling to overcome the induced prismatic deviation caused by misaligned optical centers. Common symptoms include eye strain, visual distortions, and persistent headaches.
The misalignment can also cause a feeling of visual fatigue, blurred vision that never fully clears, or even double vision (diplopia). Additionally, an improper PD can affect depth perception, making tasks like driving or participating in sports more difficult. If you experience any of these symptoms with new glasses, it is advisable to have the PD verified by an eye care professional to ensure the lenses are positioned correctly.