The Dark Focus of Visual Accommodation: Its Existence, Its Measurement, Its Effects

The dark focus or resting state of visual accommodation is that refractive state to which the eye tends to return in the absence of any visual stimulus, as in complete darkness. This tendency is reviewed in terms of past and current attempts at its quantification and its effects on visual performance. In Experiment I, a new optometer -- the polarized vernier optometer -- is described and compared experimentally to the better-known laser optometer. The former compared favorably to the latter in terms of subject acceptability and agreement of dark focus measurements. In Experiment II, the accommodative responses to Snellen letters and Landolt Cs of varying size were measured. It was observed that as the targets decreased in size and, consequently, fineness of detail, accommodation was increasingly further out. As targets became larger, accommodation shifted in toward the dark focus. In Experiment III, the dark focuses of 301 observers, varying in age and far point i.e., the amount of nearsightedness or farsightedness were measured. It was observed that the difference between the measured dark focus and the far point relative dark focus varies little in its distribution over a wide range of ages and far points. There was, however, a slight tendency for the relative dark focus to be smaller with increasing age and increasing hyperopia, and for it to be larger with increasing myopia.