What is the equation for decentration per lens?

What is the equation for decentration per lens?

• A. d = (frame PD - wearer's PD)/2
• B. d = ED - frame PD
• C. d = wearer's PD - frame PD
• D. d = frame PD + wearer's PD

Answer: (A)

Decentration per lens means shifting each lens’s optical center so the wearer's pupils align with the lens centers, compensating for any difference between the frame’s PD and the wearer’s PD. The frame PD sets how far apart the lens centers must be, while the wearer’s PD is the ideal separation between optical centers. To achieve alignment, the adjustment is split equally between the two lenses, so the amount moved on each lens is half of the difference between frame PD and wearer's PD. That gives d = (frame PD − wearer's PD) / 2. If the frame PD is larger, you decenter outward on each lens; if smaller, the shift is inward (negative). The other expressions mix quantities that don’t describe this balanced, per-lens adjustment.

Decentration per lens means shifting each lens’s optical center so the wearer's pupils align with the lens centers, compensating for any difference between the frame’s PD and the wearer’s PD. The frame PD sets how far apart the lens centers must be, while the wearer’s PD is the ideal separation between optical centers. To achieve alignment, the adjustment is split equally between the two lenses, so the amount moved on each lens is half of the difference between frame PD and wearer's PD. That gives d = (frame PD − wearer's PD) / 2. If the frame PD is larger, you decenter outward on each lens; if smaller, the shift is inward (negative). The other expressions mix quantities that don’t describe this balanced, per-lens adjustment.