Instrument myopia.

The basis for instrument myopia, the tendency to accommodate inappropriately while viewing through an optical instrument, was investigated in three experiments. The first demonstrated that the distance of a peripheral surround, analogous to a field stop, influences accommodation but that the magnitude of the effect cannot account for instrument myopia. The second experiment re-examined the hypothesis that perceived distance can affect accommodation. The data indicate that perceived distance is unlikely to influence accommodation and does not provide an explanation of instrument myopia. The last experiment tested the hypothesis that instrument myopia is a manifestation of the return of accommodation to an intermediate state of rest or equilibrium in the absence of an adequate stimulus for accommodation. Implications of the intermediate-resting-state hypothesis are discussed.