# Effects of Viewing Displays from Different Distances on Human Visual System
## 4 Discussion
[PERSON] et al. [33] observed more discomfort and fatigue with a given vergence-accommodation conflict at longer distances. This finding is in agreement with the study conducted by [PERSON] et al. [33], wherein the viewing distance had a significant effect on visual fatigue. In stereoscopic 3D displays, the accommodation distance is fixed at a particular distance from the eyes to the display screen so the viewer needs to accommodate to the screen. However, the vergence distance varies depending on the distance being simulated on the display so the viewer needs to converge at a point located in front of or behind the display plane. Moreover, [PERSON] et al. [50] and [PERSON] and [PERSON] [51] identified that accommodation and convergence may conflict in the participant viewing programs on 3D displays because of an increase in the near visual tasks, which in turn, may lead to eventual deterioration of the capability to accommodate and converge, thereby increasing the visual fatigue. Watching contents on 3D displays from short viewing distance contributes to eventual deterioration of the capability to accommodate and converge, which in turn, may lead to the development of visual fatigue. This result helps support the findings of the study conducted by [PERSON] et al. [50] and [PERSON] and [PERSON] [51]. There are other methods of 3D displays such as integral photography, which is a promising method to display 3D optical images by reproducing exactly the same light rays as emitted from real objects [52]. This method duplicates the conditions of viewing real objects. Therefore, the convergence and accommodation responses have been predicted to be consistent with the depth position of the 3D target. The accommodation response to integral 3D displays has been theoretically investigated by many researchers [53; 54; 55; 56]. The reports indicate that satisfying the super multi-view (SMV) condition is the most important requirement for obtaining a proper accommodation response. The SMV condition means that two or more light rays from the point lights of the reconstructed 3D objects reach the pupil of an observer. [PERSON] et al. [56] showed that six of the ten observers did not have an accommodation-convergence conflict in viewing integral photography in the range. Moreover, the required resolution was found to be 0.7 or more and less than 1.4 cycles per degree for inducing accommodation. [PERSON] et al. [56], concluded that integral photography can provide a natural 3D image that looks like a real object. [PERSON] and [PERSON] [57] investigated the effects of display type (2D versus 3D), viewing distance (3H versus 6H) and viewing time on visual fatigue based on percentage of maximum electromyography (EMG) contraction (%MVC) of orbicularis oculi (OO) muscle activity. They reported that viewing time and distance had significant effects on the %MVC and OO muscle activity with the details depending on the display type. The OO muscle activity of the participants watching the 3D display from a long distance (6H) did not change over the viewing time. [PERSON] and [PERSON] [52] suggested that watching 3D display from a short viewing distance contributed to the eventual deterioration of the capability to accommodate and converge. This, in turn, likely caused development of greater visual fatigue watching the 3D display as compared to watching the 2D display from an equivalent short viewing distance. The reason offered for increasing visual fatigue while viewing 3D display from short viewing distance is that doing so increases the field of view. [PERSON] and [PERSON] [58], [PERSON] et al. [59], and [PERSON] [60] showed that the reason of conflict between the accommodation and experience in 3D displays, which results in visual fatigue, is accommodation that should respond to the screen position but disparity in the two images for both the vergence stimulus of the eyes varies over time.