Photographers have known about depth of field since the beginning of photography. When a lens is focused on an object, things more in the foreground or further away may be out of focus; that range is called the depth of field. The iris in your eye is a lens too, and it adjusts to focus on near versus far objects, called accommodation. We can actually feel our iris changing its focus, and this oculomotor cue of stretching and relaxing also contributes to our depth perception.
However, using near-eye displays (in VR as well as AR), all the rendered objects are in focus, regardless of their distance perceived via parallax. Furthermore, the angle between your eyes changes when you're focused on something close up versus something further ...
