centers are placed 3–4 mm below the pupillary centers in primary gaze. The patient’s gaze will be directed much closer to the optical centers of the lenses when reading.

Contact lenses Contact lenses can be prescribed for patients with significant anisometropia that causes a symptomatic anisophoria in downgaze. Reading glasses can be worn over the contacts if the patient’s vision is presbyopic.

Refractive surgery Corneal refractive surgery may be an option for some patients with symptomatic anisometropia or anisophoria.

Occupation and Bifocal Segment

The dioptric power of a segment depends on the patient’s accommodative reserve and the working distance required for a specific job. Such focal length determinations are a characteristic not of the job but of the individual patient’s adaptation to that job. If the patient is allowed to use half of his or her available accommodation (which must be measured), the remainder of the dioptric requirement will be met by the bifocal add. For example, if the job entails proofreading at 40 cm, the dioptric requirement for that focal length is 2.50 D. If the patient’s accommodative amplitude is 2.00 D, and half of that (1.00 D) is used for the job, the balance of 1.50 D becomes the necessary bifocal add. It is essential that the accommodative range (near point to far point) be measured and that it be adequate for the job tasks.

Lens design

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 (Kryptok, Ultex type) 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