Stereoscopic images are viewed differently from real-world images, and require a level of understanding of the underlying optometrics in order to produce two images that will fuse together correctly. When viewing an object in the real world, the axes of the eyes rotate to meet at the desired location. This is called convergence, and the angle of convergence varies depending on the distance between the eyes and the object. When looking at a distant object (such as the Sun) it can be considered that the axes of the eyes are parallel due to the small angle of convergence. Accommodation is the adjustment in the focal length of the lens of the eye, allowing correct image focusing at varying distances.\n \nStereoscopic monitors need to produce a disparity in the eyes in order to fool the brain into perceiving an image at an artificial distance (which differs from the disparity that would be associated with an observation of the corresponding object in the real-world). This is achieved by producing a parallax and there are four types of parallax that can be used on an autostereoscopic monitor.\n \n[B]Zero Parallax[/B] is when the eyes converge at the same point, as happens in the real world when viewing an object. On an autostereoscopic monitor this would mean that the image is being formed on the monitor surface. In this case, the accommodation of the eyes is also at the same point as convergence and the image is comfortable to view:\n \n[CENTER][B][ATTACH=full]12[/ATTACH] [/B][/CENTER]\n[CENTER][B]Zero Parallax[/B][/CENTER]\n \n[B]Positive Parallax[/B] occurs when the eye axes are between zero parallax and parallel parallax (i.e. the eye axes are parallel as if viewing a distant object). This situation would result in an autostereoscopic image being formed behind the plane of the monitor. However, the accommodation is still at the monitor panel. This relationship between convergence and accommodation in real world viewing and can cause some discomfort in some people:\n \n[CENTER][B][ATTACH=full]11[/ATTACH] [/B][/CENTER]\n[CENTER][B]Positive Parallax[/B][/CENTER]\n \nThe association between convergence and accommodation is habitual as it is used when a person views an object in normal circumstances. Choosing the lowest possible parallax value that still gives the required sense of depth is the best method of minimising any breakdown of the stereoscopic effect.\n \n[B]Divergent parallax[/B] occurs when the eyes do not converge on an object and actually diverge as the name suggests. This unnatural eye position causes discomfort and should not be used in producing stereoscopic images. This may occur if the separation of the left and right images on the monitor exceeds that of the distance between the eyes and hence, care should be taken to choose a low value of displacement.\n[CENTER] [/CENTER]\n[CENTER][B][ATTACH=full]9[/ATTACH] [/B][/CENTER]\n[CENTER][B]Divergent Parallax[/B][/CENTER]\n \n[B]Negative parallax[/B] can be used to project images between the accommodated surface (i.e. the monitor) and the viewer’s eye position, giving the appearance of objects coming out of the monitor. The user still focuses on the screen, but the eyes converge in front of it.\n[CENTER] [/CENTER]\n[CENTER][B][ATTACH=full]10[/ATTACH] [/B][/CENTER]\n[CENTER][B]Negative Parallax[/B][/CENTER]\n \nThese parallax techniques allow the image to be correctly projected in front or behind the monitor plane, within the physical limits of eye movement. As human eyes vary in their separation distance and natural ability to move, it is important to design display software to use the lowest possible parallax so that all users remain within comfortable limits.