Multifocal Designs
Flat Top Bifocals

A flat top bifocal may be thought of as a round segment with the top cut off. In this design the optical center of the segment is located closer to the transition line, producing less image jump. Flat top segments are also called straight top bifocals and are occassionally referred to as D segments since they are shaped lik the letter D. If the lens on the previous page were a flat top design, the optical center of the segment would be only 4 mm from the transition line rather than 11 mm. The amount of prism produced at that point would only have been 0.8 instead of 2.2 which is a significant difference.

Flat Top bifocals are available if the following diameters: FT-22; FT-25; FT-28; FT-35; FT-40. The variety of sizes is possible due to the ability to manipulate the position of the optical center of the segment relative to the transition line. Since the optical center of round bifocals are always in the geometric center of the segment, they are commonly available in the relatively small 22mm diameter and are ordered as �22 Round.� Although larger diameters are available, more image jump is produced. The patient desiring a larger segment might be well advised to select another style lens.