When ordering prescription glasses, you may come across the term PD, or pupillary distance. This is the measurement (in millimeters) between the centers of your pupils and is used to ensure that the lenses are properly aligned with your eyes for optimal vision.
PD can be categorized into two types: Monocular PD, which measures the distance from the center of the nose to each pupil, and Binocular PD, which is the total distance between both pupils. For example, a right-eye PD of 30.0 mm and a left-eye PD of 32.5 mm results in a total PD of 62.5 mm.
Does PD Affect the Size of the Glasses?
It’s important to understand that PD does not change the physical size of your eyeglasses. The frame dimensions — including lens width, bridge width, and temple length — determine the actual size of the glasses on your face.
The PD simply ensures that the optical center of each lens aligns perfectly with your pupils, which is essential for:
For progressive or multifocal lenses, PD helps align multiple viewing zones, while for single-vision lenses, it centers the optical focus for a specific distance.
Clear vision
Comfort while wearing
Proper lens function, especially for progressive or multifocal lenses
Why Accurate PD Matters
Even though it doesn’t affect the frame size, providing an accurate PD ensures that your prescription works as intended. Misaligned lenses can cause eye strain, headaches, or blurry vision, particularly with progressive or high-prescription lenses.
Additionally, blurry vision or difficulty focusing can sometimes be attributed to incorrect vertical alignment, known as segment height, which determines the placement of the reading zone in progressive lenses.
Key Takeaways
PD is a measurement for lens alignment, not frame size.
It does not make the glasses physically larger or smaller.
Always provide your correct PD to ensure optimal vision and comfort.
By understanding the role of PD, you can confidently order your glasses online, knowing that the lenses will be perfectly aligned for your eyes without affecting the fit of the frames.