Where should you set the distance when measuring PD for PALs?

• A. Near
• B. Distance (infinity)
• C. Intermediate
• D. At the pupil center

Answer: (B)

The main idea is to center the lens based on where the eyes are looking most of the time in a PAL, which is distance vision. When you measure interpupillary distance for progressive lenses, you should set the distance PD with the patient gazing at a distant target—effectively at infinity. This ensures the optical centers of the lenses align with the pupils in their distance line of sight, so the distance portion of the PAL is properly centered and the wearer won’t experience unwanted prismatic effects as they look through the corridor.

Measuring at near or intermediate would shift the reference position because the pupils move slightly with gaze, causing the distance portion to be decentered and the progressive corridor to be misaligned. The idea isn’t to place the PD at the pupil center in a different gaze; it’s to establish the distance PD using a far target so the lens design matches the user’s habitual viewing, providing comfortable, accurate distance vision through the PAL.