If frame PD is 66 mm and wearer's PD is 64 mm, what is the decentration per lens?

If frame PD is 66 mm and wearer's PD is 64 mm, what is the decentration per lens?

• A. 1 mm
• B. 2 mm
• C. 0 mm
• D. 0.5 mm

Answer: (A)

The key idea is that lens decentration is used to align each lens’ optical center with the wearer’s pupil centers. When the frame’s interpupillary distance (PD) is larger than the wearer’s PD, you center the two lenses closer to each other by moving each lens inward by half of the difference. Here, the frame PD is 66 mm and the wearer’s PD is 64 mm, a difference of 2 mm. Half of that is 1 mm, so you decenter each lens 1 mm toward the bridge (nasally). This yields an effective inter-lens distance of 64 mm, matching the wearer’s PD. The other options don’t fit because they don’t produce the correct per-lens shift needed to align the optical centers with the pupil centers.

The key idea is that lens decentration is used to align each lens’ optical center with the wearer’s pupil centers. When the frame’s interpupillary distance (PD) is larger than the wearer’s PD, you center the two lenses closer to each other by moving each lens inward by half of the difference. Here, the frame PD is 66 mm and the wearer’s PD is 64 mm, a difference of 2 mm. Half of that is 1 mm, so you decenter each lens 1 mm toward the bridge (nasally). This yields an effective inter-lens distance of 64 mm, matching the wearer’s PD. The other options don’t fit because they don’t produce the correct per-lens shift needed to align the optical centers with the pupil centers.