We have proposed a method of using an aspherical phase-plate which enlarges the depth of field in order to reduce visual fatigue caused by vergence-accommodation conflict that is an issue in binocular stereoscopic displays. We have demonstrated the effectiveness of this method in stereoscopic displays and head-mounted displays (HMDs) for VR/AR. In this study, we measured the off-axis imaging characteristics of the aspherical phase-plate. We found that the image formation position depended on the off-axis angle of gaze when eyes move. Considering the measurement results and the change in the positions of the eyes, we discuss the enlargement of the depth of field.