Recall from Section 4.4 that accommodation is the process of changing the eye lens' optical power so that close objects can be brought into focus. This normally occurs with both eyes fixated on the same object, resulting in a stereoscopic view that is brought into focus. In the real world, the vergence motion of the eyes and the accommodation of the lens are tightly coupled. For example, if you place your finger cm in front of your face, then your eyes will try to increase the lens power while the eyes are strongly converging. If a lens is placed at a distance of its focal length from a screen, then with normal eyes it will always be in focus while the eye is relaxed (recall Figure 4.30). What if an object is rendered to the screen so that it appears to be only cm away? In this case, the eyes strongly converge, but they do not need to change the optical power of the eye lens. The eyes may nevertheless try to accommodate, which would have the effect of blurring the perceived image. The result is called vergence-accommodation mismatch because the stimulus provided by VR is inconsistent with the real world. Even if the eyes become accustomed to the mismatch, the user may feel extra strain or fatigue after prolonged use [246,289]. The eyes are essentially being trained to allow a new degree of freedom: Separating vergence from accommodation, rather than coupling them. New display technologies may provide some relief from this problem, but they are currently too costly and imprecise. For example, the mismatch can be greatly reduced by using eye tracking to estimate the amount of vergence and then altering the power of the optical system [4,187].
Steven M LaValle 2016-12-31