The authors measured the pupil sizes of 132 subjects and correlated these to selected designs of annular bifocal intraocular lenses (IOLs). Pupils were measured under five different testing conditions that simulated representative visual tasks: (1) driving at night, (2) reading in dim illumination, (3) reading in bright illumination, (4) distant viewing in indirect sunlight, and (5) distant viewing in direct sunlight. Pupil size diminished with age and with increased illumination. Because of accommodative pupillary constriction, there was little difference in pupil size measured while reading in bright indoor illumination and distant viewing in indirect sunlight. Evaluating a patient's pupil size permits the individualized estimation of the eye's bifocal function following implantation of each of the annular-design bifocal IOLs.