A bifocal lens may not fully satisfy all the visual needs of an older patient with limited accommodation. Even when near and distant ranges are corrected appropriately, vision is not clear in the intermediate range, approximately at arm’s length. The loss of intermediate-range vision may be exacerbated by a stronger bifocal add than is needed. This problem can be solved with trifocal spectacles, which incorporate a third segment of intermediate strength (typically one-half the power of the reading add) between the distance correction and the reading segment. The intermediate segment allows the patient to focus on objects beyond the reading distance but closer than 1 m (Clinical Example 4-4.) In contemporary practice, progressive addition lenses have supplanted trifocals in most instances.
Figure 4-25 Bifocal lens styles. n = index of refraction.
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.