Which term is defined as Frame PD minus Near PD/2?

Which term is defined as Frame PD minus Near PD/2?

• A. Inset
• B. Seg Inset
• C. Total Inset
• D. Near PD

Answer: (C)

When fitting lenses for near tasks, you need to position the near-reading portion so it lines up with the pupils. Frame PD is the distance between the frame’s optical centers, while Near PD is the pupil distance at close viewing. The amount you offset the near segment across both eyes—the total inset—is determined by how the frame spacing compares to the near spacing. This is defined as Frame PD minus half of Near PD. The reasoning is that the near PD is split between the two eyes, so half of that distance is attributed to each eye, and the remaining amount across both eyes sums to the total inset. For example, if Frame PD is 66 mm and Near PD is 60 mm, Near PD/2 is 30 mm, so the total inset is 66 − 30 = 36 mm. That means each eye’s segment would be inset by 18 mm. The per-eye portion is the seg inset, while Near PD and Frame PD describe the distances used to determine those offsets, so the term defined by that formula is the total inset.

When fitting lenses for near tasks, you need to position the near-reading portion so it lines up with the pupils. Frame PD is the distance between the frame’s optical centers, while Near PD is the pupil distance at close viewing. The amount you offset the near segment across both eyes—the total inset—is determined by how the frame spacing compares to the near spacing. This is defined as Frame PD minus half of Near PD. The reasoning is that the near PD is split between the two eyes, so half of that distance is attributed to each eye, and the remaining amount across both eyes sums to the total inset. For example, if Frame PD is 66 mm and Near PD is 60 mm, Near PD/2 is 30 mm, so the total inset is 66 − 30 = 36 mm. That means each eye’s segment would be inset by 18 mm. The per-eye portion is the seg inset, while Near PD and Frame PD describe the distances used to determine those offsets, so the term defined by that formula is the total inset.