Welcome to brain Stuff from house stuffworks dot com where smart happens. Hi. I'm Marc Brain with today's question, why do three D movies and three D televisions give lots of people headaches? The vast majority of people can go watch a normal two D movie, or can lie on their couch and watch their normal two D television set without any major problems. They can watch for hours, and in general, the vast majority of people don't get a headache from doing that. But three D movies are different.
Three D movies cause a lot of people to get headaches. And even if you don't get that headache, twenty minutes into the movie, you may notice that your eyes don't feel quite right. It could be anything from excessive tearing to an achiness that makes you take off the glasses and rub your eyes every ten minute. It's have you ever wondered what causes this effect? A letter from Walter Merch, who was a famous film editor, to Roger Ebert tries to get its arms around what causes this problem. He
calls it a convergence focus issue. You can understand the problem in the following way. Hold your hand about three inches away from your nose, and look at it. Two things are happening. First, the muscles that control your eyeballs are rotating your eyeballs, so your pupils are pointing towards your nose. If you were to look at someone who's looking at his or her hand three inches away from his or her face, the person would look cross eyed. The point of convergence is the distance of the object
away from your face. Now, when you're holding your hand three inches away from your face, you're exaggerating this effect, but the point of convergence is happening no matter what you're looking at. Second, the muscles that control the lenses in your eyes are manipulating the lens is so that you can focus on something so close to your face. Again, it's exaggerated here, but the focus distance has to do with how far away an object is when you look
at it. This is completely normal any time you look at anything in the real world. The muscles controlling the convergence point of your eyeballs and the muscles controlling the lenses are working in harmony. The two distances are the same. In a three D movie, however, you have to do something different. The focal length is constant. Your eyes have to stay focused on the plane of the screen or
on the television set screen. Meanwhile, you have to change the convergence angle constantly to get the three D effect to work. This is not the way the system expects the world to be, and it takes extra effort on the part of your brain to make this happen. The letter from Walter Merch points out that there's another side effect to this problem besides the headaches that many people get. It takes your eyes and your brain some amount of time to figure out what the right convergence angle is
for the effect of each scene. So some scenes are wanting you to see a three D effect that's nearer your face, some want you to see a three D effect that's far away, and so on. Your brain because this is a little weird, is taking time to get the convergence angle right for each different scene. This means that three D films affect the film editing art. Scenes can't be cut together as quickly as they can in a two D movie, or they would confuse your brain
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