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 away, called vergence. So, we get mixed signals, focus (accommodation) on one distance and vergence at another. This results in what is called an accommodation-vergence conflict. This disparity can become tiring, at best, and inhibits the illusion of realism. This is a problem with both wearable AR and VR devices.

Potential solutions may emerge using eye tracking to adjust the rendered image according to your vergence. There is also the promise of advanced light-field technology that more accurately merges computer-generated graphics with real-world light patterns (see Magic Leap at https://www.magicleap.com).