The visual function of lens accommodation was measured while subjects used stereoscopic vision with a head mounted display (HMD). Eyesight while viewing stereoscopic Landolt ring images displayed on HMD was also studied. Accommodation to virtual objects was seen when subjects viewed stereoscopic images of 3D computer graphics, but not when the images were displayed without appropriate binocular parallax. This suggests that stereoscopic moving images on HMD induced visual accommodation. Accommodation should be adjusted to the position of virtual stereoscopic images induced by parallax. A difference in the distances of the focused display and stereoscopic image may cause visual load. However, an experiment showed that Landolt rings of almost the same size were distinguished regardless of virtual distance of 3D images if the parallax was not larger than the fusional upper limit. The results of this study suggest that stereoscopic moving images on HMD induced visual accommodation by expansion and contraction of the ciliary muscle, which was synchronized with convergence. Appropriate parallax of stereoscopic vision should not reduce the visibility of stereoscopic virtual objects.
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