If you have recently received an eye prescription and noticed that the Pupillary Distance (PD) measurement is missing, you are not alone. This number is a common omission, particularly when prescriptions are delivered digitally or printed for the patient. The absence of this measurement can be confusing, especially when attempting to purchase new eyewear online, where the PD is a required field. This measurement dictates how your lenses are manufactured and placed within your chosen frames. Understanding the purpose of this number and the reasons for its omission will help you navigate the process of obtaining new glasses.
The Role of Pupillary Distance
Pupillary Distance (PD) is the measurement in millimeters of the space between the centers of your two pupils. This figure is used by the optician or lab to ensure that the optical center of each lens aligns precisely with the center of your eye’s pupil. Proper alignment is necessary because the prescription correction is strongest and clearest at the lens’s optical center. The average adult PD typically falls between 54 mm and 74 mm.
If the PD used is incorrect, the light passing through the lenses will be misdirected, creating an unwanted optical effect known as induced prism. Even a small error can lead to issues, especially with stronger prescriptions. These issues often manifest as eye strain, headaches, blurred vision, and reduced depth perception. For optimal visual clarity and comfort, the PD must be accurate.
Understanding Why PD is Absent on Prescriptions
The primary reason PD is often not included on the prescription form is a professional distinction between prescribing and dispensing eyewear. The eye doctor determines the refractive error—the actual power of the lens needed—which constitutes the prescription. The PD, however, is considered a “dispensing” measurement, a physical fitting parameter taken by the optician when the frame is selected. Since the PD can vary slightly depending on the frame size and shape, some practitioners argue it should be measured only at the point of sale.
Another factor is a business practice designed to encourage in-house purchases. By omitting the PD, the optical provider retains control, making it more difficult for patients to purchase frames from a competitor or an online retailer. Additionally, some practitioners cite liability concerns. If the doctor provides the PD and an online vendor uses it incorrectly, the prescribing doctor could potentially be held responsible for the poor visual outcome.
Practical Ways to Obtain Your Measurement
Since the PD is necessary for ordering glasses, there are several ways to obtain the figure. The most direct method is to simply ask your prescribing office for the measurement. While they may initially decline or charge a small fee, many offices will release the number upon request, as they likely have the PD on file from previous fittings.
If your request is denied, you can seek a professional measurement from any optical shop or dispensing optician. They can use a specialized device called a pupillometer to take a precise measurement, often for a minimal fee or sometimes for free. For a quick solution, you can also take the measurement yourself at home using a millimeter ruler and a mirror or a friend.
Modern technology also offers convenient alternatives through smartphone apps and online tools. These tools use your phone’s camera and a standard card for scale to calculate your PD with good accuracy. When measuring, you may encounter a single number (e.g., 62), which is the total distance, or a dual PD (e.g., 31/31). The dual PD separates the distance from the center of each pupil to the bridge of your nose and is preferred for higher prescriptions or multifocal lenses.