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The problem occurs because our eyes have evolved 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 [[http://doc-ok.org/?p=1360 HMDdesign]] 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.
