On proximal effects in objective and subjective testing of dark accommodation

This study investigated to what extent objective and subjective measurements of accommodation in darkness are affected by instrumentation or by flashed light stimuli that are arranged close to the subject's eye, and by the instructions given to the subjects. Subjects differed markedly in their susceptibility to these factors. Dark accommodation measured with an infra-red autorefractometer tended to be more distant by 0.2 D on the average if the autorefractometer allowed a free view into a large, but dark, room compared with a closed apparatus. When subjects were instructed to look into far or near distances during objective measurements, mean dark accommodation was more distant by 0.15 D and closer by 1.4 D, respectively, relative to the condition of 'relaxed eyes'. To find a simple screening test of dark accommodation, a 'number test' was investigated: subjects had to report which was the most sharply visible among a series of numbers that were simultaneously flashed at different optical distances in a Badal optometer. Objective measurements revealed that this test induced near shifts of 0.37 D on the average, relative to accommodation in a dark surround. Therefore, the results of the number test were only moderately correlated to objective measurements.