Changes in astigmatism with accommodation.

An experiment is described in which changes in ocular astigmatism were recorded over an accommodation stimulus range of 0-4 D, in the right eyes of a group of 31 normal subjects aged 17-37 years. The visual axes were horizontal and the observation conditions were monocular. Although subjects had a range of spherical ametropia, all had initial astigmatism of 1.0 DC or less. It was found that, for most subjects in the group, astigmatism changed in the with-the-rule direction, the mean change being -0.036 DC at axis 176 deg for each dioptre of accommodation response. It is speculated that these changes might reflect increases in lens tilt about a horizontal axis, caused by the combined effects of a slacker zonule and gravity.