For aspherics, should you measure monocular or binocular PDs?

• A. Monocular PDs
• B. Binocular PDs
• C. Neither PDs nor any measurement is needed
• D. Pelvic PDs

Answer: (A)

Precise centering of aspheric lenses relies on aligning the optical center for each eye with that eye’s visual axis. Each eye has its own pupil position, and the two eyes are not perfectly symmetric. Using a single binocular PD assumes a shared center between eyes, which can misplace the optical centers for one eye and introduce unwanted prismatic effects and peripheral blur in an aspheric design. Measuring monocular PDs ensures the lens is centered correctly for each eye, preserving the intended performance of the aspheric surface, especially with higher add powers or complex decentrations. That’s why monocular PDs are the appropriate measurement.