If frame PD equals wearer's PD, what is the decentration per lens?

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

Answer: (A)

Decentration per lens is how far each lens must be shifted horizontally so the optical center aligns with the wearer's pupil. It depends on the difference between frame PD and the wearer's PD, usually calculated as half of that difference: (frame PD − wearer's PD) ÷ 2.

If the frame PD equals the wearer's PD, the difference is zero, so no decentration is needed. The optical centers already line up with the pupils, giving a decentration of 0 mm. If there were a mismatch, you would decenter each lens by half the difference, toward the appropriate side.