There are several possibilities, so knowing the cause for a particular person will only be possible after a thorough consultation at an eye clinic.
Having said that, what are the possibilities?
One is eye dryness. When we read or look at screens (eg watching movies, playing games) for prolonged periods of time, we tend not to blink as often. This means the eyes are exposed to the air for longer periods of time and the tears will evaporate more, leading to a dry ocular surface. Although the surface of the eye may look smooth and shiny, in reality there are microscopic irregularities that affect focusing-and these irregularities are exposed or may get worse whenever the eyes get very dry.
Another possibility, depending on age, is the effect of presbyopia or Lao Hua. With age, the lens in the eye becomes stiffer and does not autofocus as quickly. This means that sometimes after looking at near things for a long time, the eye takes a while to adjust for distance. Distance things look blurry until the lens finally adjusts its focus.
Even without presbyopia, younger people can get a 'spasm' of the ciliary muscle that controls the focusing of the lens and eye. The feeling is quite similar, in that distance vision gets blurry for a while until the ciliary muscle finally relaxes.
Just to re-iterate, if there are any ongoing problems with blurred vision, it is best to consult a doctor. This answer is not a substitute for such a consultation.
