The Effect of Pupil Size on Accommodation, Convergence, and the AC/A Ratio

¹ Department of Ophthalmology, New York University School of Medicine.

The present report describes the effect of pupil size on the accommodation-convergence relationship. Measurements of accommodation and associated changes in convergence were made with a Fincham coincidence optometer and modified haploscope. Responses ivere obtained over a wide range of stimulus levels with the observer viewing the target through a graded series of artificial pupils. Increasing the dioptric stimulus to accommodation resulted in a nearly linear increase in both the accommodative response and associated vergence. However, with pupil sizes of 1.0 mm. or less there was a reduction in the slope of these stimulusresponse functions. There was, accordingly, a significant decrease in the stimulus (clinical) AC/A ratio for these pupil diameters. The response AC/A ratio, on the other hand, toas not significantly influenced by variation in pupil size. Since pupil diameters of 1.0 mm. or less are not usually attained with parasympathomimetics in treating accommodative eso deviations, effective control cannot be attributed solely to an increase in depth of focus resulting from pupillary constriction. When the target, imaged at the far point of the eye, was viewed through a 0.5 mm. pupil, some subjects exhibited an increase in accommodation. This effect appears related to the increase in refractive state of the eye that occurs in the absence of an optical stimulus to accommodation.