Inset is the distance from the geometric center to the MRP. Which expression represents Inset?

• A. Frame PD - Distance PD/2
• B. Distance PD - Near PD/2
• C. Frame PD - Near PD/2
• D. Near PD

Answer: (A)

Inset is the horizontal distance from the frame’s geometric center to the near reference point (MRP). To position the near reference correctly, you account for how wide the frame is (frame PD) and how far apart the wearer’s pupils are (distance PD). The MRP sits toward the temporal side by roughly half the wearer’s PD from the midline, so the distance from the frame center to the MRP equals the frame PD minus half of the wearer’s PD. That gives inset = frame PD − distance PD/2. The other options don’t reflect this geometric relationship between the frame’s width and the wearer’s interpupillary distance.