The aim was to compare fixation distances between accommodation and convergence in young and middle-aged subjects while they viewed 2D and 3D video clips. Measurements were made using an original machine, and 2D and 3D video clips were presented using a liquid crystal shutter system. We developed an original machine by combining WAM-5500® and EMR-9® to perform the measurements. As results, subjects’ accommodation and convergence were found to change the diopter value periodically when viewing 3D images. When subjects are young, accommodative power while viewing 3D images is similar to the distance of convergence, while subjects are middle-aged, their accommodation is weak. It is generally explained to the public that, "During 3D vision, while accommodation is fixed on the display that shows the 3D image, convergence is at the stereoimage”. According to the present findings, however, such explanations are mistaken.