Accommodation and Vergence

One of the troubles with 3D movies is that vergence and accommodation are linked together. This means that when you focus on something close it causes your eye to cross and vice-versa. So, when you are watching a 3D movie and you see something "close" and cross your eyes too look at it your eyes automatically focus. The problem is that the image is still projected in the distance so you are trying to focus up close on an object that is still in the distance. For a full discourse on the subject click here.