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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 focusing. 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.
