Seg inset is the distance from the MRP to the center of a bifocal seg. Which expression is commonly used as an alias for Seg inset?

Seg inset is the distance from the MRP to the center of a bifocal seg. Which expression is commonly used as an alias for Seg inset?

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

Answer: (A)

Seg inset shows how far inward from the MRP the center of the bifocal segment must sit so the near vision line aligns with the eye. In practice, this offset is commonly estimated using interpupillary distance values: distance PD as the baseline reference and near PD to account for the inward shift needed for near viewing. Taking distance PD and subtracting half of near PD gives a practical, widely used expression for that inset. It reflects using the distance reference while incorporating the near convergence effect through half of the near PD. Using frame PD as the base or using the full difference without the half near PD doesn’t align with the common quick-rule approach practitioners use, which is why distance PD minus half of near PD is the standard alias.

Seg inset shows how far inward from the MRP the center of the bifocal segment must sit so the near vision line aligns with the eye. In practice, this offset is commonly estimated using interpupillary distance values: distance PD as the baseline reference and near PD to account for the inward shift needed for near viewing. Taking distance PD and subtracting half of near PD gives a practical, widely used expression for that inset. It reflects using the distance reference while incorporating the near convergence effect through half of the near PD. Using frame PD as the base or using the full difference without the half near PD doesn’t align with the common quick-rule approach practitioners use, which is why distance PD minus half of near PD is the standard alias.