I was one of those smartasses who made fun of people who complained about getting headaches when they watched 3D movies, until I saw Green Hornet. I left that thing with a brainsplitter so intense that I couldn’t sleep well for two days. Thinking back on the experience, I formulated a theory about what was causing the headaches. Now, I’m no scientist, so I could be wrong. But my theory makes sense to me — and, besides, I tested it on Thor, and it seemed to work. For me at least.
The problem I had with Green Hornet was that I kept trying to focus on things that the camera didn’t want me to focus on. Say there’s a scene where two characters are sitting in front of a bookshelf. In real life, if you’re bored with the conversation, you might turn your eyes to the bookshelf and try to read the titles on the spines. I might do that, anyway. When you try to do that watching a movie, though, the titles on the spines are blurry, because the camera isn’t focused on them. We’re used to that in 2D movies. In 3D, the temptation is to try to adjust the focus on your own, using the same eyeball tricks you use in real life. It doesn’t work, and the attempt causes the headaches.
I watched Thor like I was being tested by evil scientists, where my goal was to always find the area of the screen in sharpest focus, and look only at that.
I think the reason I never encountered this problem before was that most of the 3D movies I had seen were less boring than Green Hornet, so I had never been tempted to investigate the out-of-focus parts of the screen.