To match the segment height to an old bifocal if it is 3 mm below the lower lid margin, where should the new segment be placed?

The optimal placement for the new bifocal segment is to position it 3 mm below the lower lid margin, matching the height of the previous bifocal. This ensures that the new lenses will provide the same visual experience as the old ones, maintaining the correct alignment for reading and other near tasks.

By placing the new segment at the same measured height, the user is likely to experience comfortable and effective bifocal use without having to adjust to a different segment height, which could cause visual strain or disruption in their line of sight. This practice emphasizes consistency in the fitting of lenses for patients transitioning from old to new eyewear. Maintaining the same segment height safeguards against potential issues related to line of sight, especially crucial in multifocal prescriptions. Keeping the segment height consistent will ultimately support the wearer’s visual habits and demands.