Vergence-accommodation conflict, also known as VAC, or Accommodation-vergence conflict, occurs when your brain receives mismatching cues between the distance of a virtual 3D object (vergence), and the focusing distance (accomodation) required for the eyes to focus on that object. This occurs while looking at stereoscopic imagery, such as watching 3D TV/cinema, as well as in all current, traditional HMDs.
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 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 evolved an 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 accommodation and 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.
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.
- Oliver Kreylos, Accommodation and Vergence in Head-mounted Displays
- Gregory Kramida and Amitabh Varshney, Resolving the Vergence-Accommodation Conflict in Head Mounted Displays
- Oliver Kreylos, Head-mounted Displays and Lenses