Night myopia and the intermediate dark focus of accommodation*

The phenomenon of night myopia, the tendency to overaccommodate for distant objects as luminance is decreased, results from the passive return of accommodation to an individually determined intermediate resting or dark focus. More generally, accommodation is viewed as a compromise between the subject's individual resting focus and the accommodative stimulus. Under optimum viewing conditions, accommodation tends to correspond to the distance of the stimulus, but is biased progressively toward the dark focus as the adequacy of the accommodative stimulus is degraded by decreased luminance. Control experiments suggest that optical aberrations are not major factors that contribute to this effect.