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

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

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

Answer: (B)

When customizing bifocal or multifocal lenses, the near portion must line up with the eye’s reading axis, which is slightly different from the distance axis. The distance PD is the interpupillary distance used for distance vision, and the near PD is the PD applicable for near work. Because the two PDs typically differ, the near segment needs to be shifted inward a bit on each lens so that, when looking at near objects, the pupil is centered over the correct part of the lens. The amount of that horizontal shift, called the seg inset, is defined as half of the difference between distance PD and near PD. In other words, seg inset = (Distance PD − Near PD) / 2. This symmetric inset on both eyes places the near portion so that the wearer’s reading axis aligns with the near PD, ensuring comfortable and accurate vision at close distances. If the distance and near PDs are identical, no inset is needed. If the near PD is smaller than the distance PD, the seg inset is a positive value, moving the near segment inward by half the difference. This is why the correct term for that measurement is seg inset.

When customizing bifocal or multifocal lenses, the near portion must line up with the eye’s reading axis, which is slightly different from the distance axis. The distance PD is the interpupillary distance used for distance vision, and the near PD is the PD applicable for near work. Because the two PDs typically differ, the near segment needs to be shifted inward a bit on each lens so that, when looking at near objects, the pupil is centered over the correct part of the lens.

The amount of that horizontal shift, called the seg inset, is defined as half of the difference between distance PD and near PD. In other words, seg inset = (Distance PD − Near PD) / 2. This symmetric inset on both eyes places the near portion so that the wearer’s reading axis aligns with the near PD, ensuring comfortable and accurate vision at close distances.

If the distance and near PDs are identical, no inset is needed. If the near PD is smaller than the distance PD, the seg inset is a positive value, moving the near segment inward by half the difference. This is why the correct term for that measurement is seg inset.