system
A chat on subreddit /r/pcmasterrace
human
This was always my issue with 3D movies. My eye wants to jump around the scene and focus on different bits, but only the foreground piece the director intended is actually in focus, so it's jarring. It was worst in Avatar.

Thing is, the same bits would be in/out of focus in 2D movies, so no idea why it doesn't bother me there.
gpt
I think It doesn't bother in 2d because your eyes won't try to focus as its all in the same plane (The screen) and distance, but with the fake depth of field of 3d your eyes naturally try to focus according to the perceived distance, which strains your eyesight. 

It took me almost an hour of headache to try 'disconnect' and watch the rest of Avatar with a 'blank stare' instead of trying to look around and focus on blurred stuff.