Human factors associated with 3D stereo-imaging systems have been reported in a number of previous studies [
4]. Sources of visual discomfort while staring at the stereoscopic image include 1) excessive binocular disparities (vertical/horizontal/crossed/uncrossed disparity), 2) vergence-accommodation mismatch (conflict), 3) imperfect image separation (interocular crosstalk) [
5], and 4) imperfect 3D rendering. Among these factors, in this article, we mainly focus on the vergence-accommodation mismatch and the resultant visual fatigue [
6,
7]. When a person stares at an object in the real world, the visual lines of the left and right eyes cross at the object and the focal distance and the vergence distances become identical. When staring at an object on a 3D display, however, the point at which the two visual lines cross is not the same as the focal points of the eyes. Consequently, the focal distance and the vergence distance are different from each other (
Figure 2). This vergence-accommodation mismatch frequently reduces the viewer's ability to fuse the binocular stimulus and induces sensory conflict and accommodative asthenopia that result in visual fatigue [
8,
9]. Visual fatigue from viewing stereoscopic images includes a wide range of visual symptoms, including eyestrain, dried mucus or tears around the eyelids, the feeling of pressure in the eyes, pain around the eyes, discomfort when the eyes are open, burning eyes, difficulty in focusing or blurred vision, stiff shoulders, and headaches [
10]. Causes of visual fatigue are thought to be as follows: 1) geometrical distortions between two images, 2) differences in electrical characteristics between two images, 3) conflicts between convergence eye movement and the accommodation function, and 4) excessive binocular parallax [
11]. Causes 1) and 2) are related to the performance of the electric equipment utilized for stereoscopic images and therefore, can be prevented by improving the quality of the images using high-performance video equipment. However, causes 3) and 4) are related to a basic problem of binocular fusion in the brain, and the detailed physiological mechanism of this fusion is not well understood. The visual fatigue from viewing stereoscopic images is not so serious when the stereoscopic images are displayed within the depth of focus; however, the fatigue becomes serious outside the depth of focus.