Accommodation toward diameter change of a spotlight in a dark room

The three-dimensional optometer (TDO III) was used to measure accommodative responses when gazing at a spotlight that changed its diameter in an otherwise completely dark room. Three emmetropic females served as subjects and the stimuli were presented at 33 cm (-3 D) from the subjects. The subjects felt that the spotlight approached them when its diameter increased. However, the magnitude of accommodation when viewing the smaller diameter spotlight was greater than that for the larger diameter spotlight. This means that the eye accommodated less, although the spotlight was felt to be nearer, and contradicts the common belief based on the subjectively measured data reported earlier. By conducting several supplemental experiments, we infer that this phenomenon comes from the effort in reducing accommodation lag dynamically in order to see the smaller target more clearly.