The most important characteristic of the bifocal segment is the segment height in relation to the patient’s pupillary center. The lenses will be unsuitable if the segment is placed too high or too low for the specific occupational need.
Segment width is substantially less important. The popular impression that very large bifocal lenses mean better reading capability is not supported by projection measurements. At a 40-cm reading distance, a 25-mm flat-top segment provides a horizontal reading field of 50–55 cm.
At a 40-cm distance, an individual habitually uses face rotation to increase his or her fixation field when it exceeds 45 cm (30° of arc); therefore, a 25-mm-wide segment is more than adequate for all but a few special occupations, such as a graphic artist or an architectural drafter using a drawing board. Furthermore, with a 35-mm segment producing a horizontal field 75 cm wide, the focal length at the extremes of the fixation field would be 55 cm, not 40 cm! Therefore, the split bifocal is useful not because it is a wider bifocal lens but because of its monocentric construction.
The shape of the segment must also be considered. For example, round-top segments require the user to look far enough down in the segment to employ his or her maximum horizontal dimension. In addition, these segments exaggerate image jump, especially in myopic corrections.
Segment decentration To avoid inducing a base-out prism effect when the bifocal lens–wearing patient converges for near-vision tasks, the reading segment is generally decentered inward. This design is especially important in aphakic spectacles. Consider the following points for proper decentration:
Working distance. Because the convergence requirement increases as the focal length decreases, additional inward decentration of the bifocal segment is required.
Interpupillary distance. The wider the interpupillary distance, the greater the convergence requirement and, correspondingly, the need for inward decentration of the segments.
Lens power. If the distance lens is a high-plus lens, it will create a greater base-out prism effect (ie, induced exophoria) as the viewer converges. Additional inward decentration of the segments may be helpful. The reverse is true for high-minus lenses.
Existing heterophoria. As with lens-induced phorias, the presence of an existing exophoria suggests that increasing the inward decentration would be effective. An esophoria calls for the opposite approach.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.