Overaccommodation, Underaccommodation, and the Clinical Phoropter: A Study of Instrument Myopia and Related Effects

ABSTRACT To explore the degree to which a phoropter can induce overaccommodation (instrument myopia) and/or underaccommodation, 19 college‐age subjects participated in a series of measurements of accommodation response. Accommodation responses were assessed while subjects looked through a phoropter (both with standard 19‐mm viewing apertures and through 2‐mm artificial pupils) and when no phoropter was present. Subjects were tested both monocularly and binocularly, and under three stimulus conditions (near target, far target, and total darkness). Even without the phoropter, subjects tended to overaccommodate for the far target. The 19‐mm phoropter produced no significant overaccommodation or underaccommodation effects over and above those observed without the presence of the phoropter. The 2‐mm artificial pupils produced significantly more underaccommodation for the near target. Results indicate a tendency for accommodation response to approach an intermediate resting position as the viewing situation becomes impoverished.