Vergence-accommodation conflict

In normal conditions the human visual system expects vergence and accommodation distances to match.

The human visual system has not evolved to view these types of artificial 3D images comfortably, so VAC can be a very unpleasant sensation for the viewer.

It can cause visual fatigue and headaches after a short period of time; It is one of the main contributors to virtual reality sickness.

[5] People playing 3D video games have often reported eye strain afterward, or that the 3D effect is disorienting.

Even though this is the case, users of classic stereoscopic devices report being unable to look at the 3D screen for a long period of time.

For example, a virtual object by means of binocular disparity can be placed at a 30 cm distance, corresponding to 3.33 Diopters.

Popular examples of these devices include the Oculus Quest 2, HTC Vive, Valve Index and the Microsoft HoloLens.

[9] VAC can also be experienced when using other technologies, including: VR and AR hardware companies often ask software developers not to show virtual content too close to the user in the devices.

In VR and AR, new types of displays have been developed since the 2010s that can minimize or eliminate VAC to non-issue levels.

The basic principle relies on dynamically adjusting focal distance of displays based on the gaze direction.

Modulation of a focal distance can be, for example, physical actuation of the screen in relation to a fixed eyepiece optics, alternatively it can be utilization of varifocal[23][24] lens element(s).

While varifocal approach mitigates or entirely solves VAC, it cannot convey realistic monocular focus cues.

This gives the ability to accommodate eyes within the available range of focal distances and perceive realistic monocular focus (image blur) cues similarly to natural viewing conditions.

[28] However, SeeReal's solution requires eye tracking, which can limit the 3D capabilities of the displays such as the field of view of the 3D effect.