Which bifocal type allows the optical center to be placed on the segment line?

The Franklin-type or Executive bifocal lens is specifically designed to have a continuous segment that goes across the entire bottom part of the lens, allowing for the optical center to be aligned with the segment line. This design facilitates a seamless transition between the distance vision portion and the near vision segment, which is particularly advantageous for patients who require both distance and near correction without any significant disruption in their field of vision.

The clarity provided across the entire lower section can be very beneficial for tasks that require frequent switching between distance and near vision, such as reading or computer work. This characteristic distinguishes it from other bifocal designs, where the optical center is not always aligned with the segment line, potentially affecting vision effectiveness and comfort for the wearer.

Other bifocal types, such as round segments and flat tops, have different configurations that sometimes result in the optical center being above or at a different position relative to the segment line, which can lead to a less optimal experience for certain tasks. Therefore, the inseparable design of the Executive lens that allows proper alignment with the segment is its defining feature.