Pupillary distance (PD) is the essential measurement of the space between the centers of your pupils, ensuring your eyes align perfectly with the optical center of your lenses for maximum visual clarity. An inaccurate PD can lead to eye strain, headaches, and even double vision, as the lenses fail to provide the proper correction where your eyes need it most. Discover why professional fitting is vital—especially for multifocals and high-powered prescriptions.