Accommodative hysteresis as a function of target--dark focus separation.

The concept of dark focus (DF) as fulcrum for accommodative effort was examined by inducing adaptation of tonic accommodation with the target at varying dioptric distances from the DF. Mean pre- to post-changes in DF at the initial post fixation test essentially were linear (r2 = 0.975), with zero intercept, with respect to the target-DF separation, as were the data of most individual subjects. Target-DF distance thus is a major determinant of adaptation of tonic accommodation. These results are consistent with the interpretation of DF as a zero reference point or fulcrum about which accommodative effort varies.