||When viewing a real object, our eyes will accommodate to focus at the object. At the same time, our eyes will also verge so that the visual axis of both eyes will point towards that object. In other words, the vergence of eyes will change in accordance to the accommodation of the eyes. However this relationship may be disturbed when viewing stereo images with changing stereo depths presented on a binocular display with fixed lens focus. The incompatibility between the varying stereo depth and the fixed lens focus can post an unnatural demand on human vergence eye movements. This may cause eye fatigue. An experiment has been conducted to investigate the levels of eye fatigue caused by viewing stereo images with changing stereo depths on a binocular display with compatible and incompatible lens focuses. Four viewing conditions have been studied and they exhaust the factorial combinations of two lens focus (40cm and 200cm) and two changing stereo depth cues (40cm ± 0.3 dioptres and 200cm ± 0.3 dioptres). This design examines the effects of lens focus, stero depth, and their interactions (i.e., compatibility between lens focus and stereo depth). In this study, we are interested in the compatibility effects when the lens focus or stereo depth is controlled at 40cm and 200cm, respectively. When using a binocular display with fixed lens focus of 200cm, viewing stereo images with incompatible stereo depths (40cm) can significantly increase the rated levels of eye fatigue (p<0.001, F1,383 = 33, ANOVA). This is an important finding and carries serious implications to those manufacturers of binocular displays with fixed lens focus of 200cm (e.g., iMD Ltd.). Unfortunately, adjusting the lens focus of the displays to 40cm to match the stereo depths of the images did not result in significant reduction in eye fatigue (p>0.5, F1,383 = 0.36, ANOVA). The lack of benefits is due to an opposite confounding effects of accommodation on eye fatigue: as the lens focus is reduced to match with the stereo depth, the accommodative demand on the eyes is increased. The former reduces the levels of eye fatigue while the latter does the opposite. Data also indicate that when the confounding effect of accommodation is in agreement with the compatibility effects between lens focus and stereo depth, very significant reduction in rated levels of eye fatigue are reported when lens focus is changed from 40cm to 200cm in order to match with images of stereo depths of 200cm ((p<0.001, F1,383 = 55.5, ANOVA). Further discussion and implications to the design of binocular displays can be found in the thesis.