View source for Vergence-accommodation conflict

[[File:Vergence accommodation conflict image.png|thumb|right|250px|Vergence-accommodation conflict in VR compared to real-world vision]]
A '''vergence-accommodation conflict''' ('''VAC'''), also known as an '''accommodation-vergence conflict''', is a phenomenon in the human visual system that occurs if the brain receives mismatching cues between [[vergence]] and [[Accommodation (eye)|accommodation of the eye]]. This commonly occurs in [[virtual reality]] devices, [[augmented reality]] devices, and binocular flat focus autostereoscopic displays. The effect can be unpleasant and cause eye strain.

It can contribute to focusing problems, visual fatigue, and eyestrain, while looking at stereoscopic imagery, and vision effects that linger even after ceasing looking at the imagery.

In traditional stereoscopic technologies, the virtual image is focused at a fixed depth away from the
eyes, while the depth of the virtual objects, and the amount of eye convergence, varies depending upon the content (see [http://doc-ok.org/?p=1360 HMD optical design]).

The problem is less severe in 3D TV/cinema, when it is properly taken into account during content creation and display. Part of the reason it's less severe, is that the 3D objects are not close to the viewer.

The problem occurs because our eyes have an [https://en.wikipedia.org/wiki/Accommodation_reflex accommodation-vergence reflex], which trains them to automatically adjust their optical focus (accommodation) based on the perceived distance to the objects (vergence) that they are looking at. This helps make focusing quick and accurate.

When a virtual object appears to be mere inches in front of the user’s face, but the image of that object is, optically, several meters away (as it is in common HMD design), the user’s eyes may focus on the wrong distance, causing the virtual object to appear blurry. The same can happen when the virtual object is very far away, but the effect is less pronounced.

A person's eyes can adapt to this conflict while looking at stereoscopic imagery, leading to [https://en.wikipedia.org/wiki/Accommodation_(eye) accommodation] and [https://en.wikipedia.org/wiki/Vergence vergence] temporarily decoupling. At that point other focusing reflexes take over, and focusing tends to improve. Although it may be difficult to rapidly re-focus on some virtual objects, since those other focusing reflexes can be slower.

Closing one eye is one method for focusing on a nearby virtual object, if the user is having problems. With one eye closed, there is no conflict between [https://en.wikipedia.org/wiki/Vergence vergence] and the required [https://en.wikipedia.org/wiki/Accommodation_(eye) accommodation].

The natural accommodation-vergence coupling will re-establish at some point after taking off the [[HMD]]. Users might experience strange-feeling vision problems until it does.

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==References==

* Oliver Kreylos, [http://doc-ok.org/?p=1602 Accommodation and Vergence in Head-mounted Displays]

* Gregory Kramida and Amitabh Varshney, [https://www.cs.umd.edu/sites/default/files/scholarly_papers/Kramidarev.pdf Resolving the Vergence-Accommodation Conflict in Head Mounted Displays]

* Oliver Kreylos, [http://doc-ok.org/?p=1360 Head-mounted Displays and Lenses]

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