If you know Frame PD and Near PD, which expression gives Total Inset?

If you know Frame PD and Near PD, which expression gives Total Inset?

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

Answer: (A)

Total inset is about adjusting the lens centers so they align with the eyes for near vision. The frame PD fixes how far apart the two lens centers are in the frame, while the near PD tells you how close the eyes come together when viewing something near. To place the near portion of the lenses correctly, you move each lens inward by half the amount needed to go from the frame’s spacing to the near spacing. Doing that across both lenses leads to taking the frame distance and subtracting half of the near distance, which gives the total inset expression used in this context. Using the distance PD would reflect far-vision spacing, not near, and using near PD alone wouldn’t account for how the frame’s geometry interacts with near alignment.

Total inset is about adjusting the lens centers so they align with the eyes for near vision. The frame PD fixes how far apart the two lens centers are in the frame, while the near PD tells you how close the eyes come together when viewing something near. To place the near portion of the lenses correctly, you move each lens inward by half the amount needed to go from the frame’s spacing to the near spacing. Doing that across both lenses leads to taking the frame distance and subtracting half of the near distance, which gives the total inset expression used in this context. Using the distance PD would reflect far-vision spacing, not near, and using near PD alone wouldn’t account for how the frame’s geometry interacts with near alignment.