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Speaker 1: Welcome to brain Stuff from house stuffworks dot com where

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Speaker 1: smart happens. Hi. I'm Marc Brain with today's question, why

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Speaker 1: do three D movies and three D televisions give lots

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Speaker 1: of people headaches? The vast majority of people can go

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Speaker 1: watch a normal two D movie, or can lie on

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Speaker 1: their couch and watch their normal two D television set

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Speaker 1: without any major problems. They can watch for hours, and

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Speaker 1: in general, the vast majority of people don't get a

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Speaker 1: headache from doing that. But three D movies are different.

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Speaker 1: Three D movies cause a lot of people to get headaches.

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Speaker 1: And even if you don't get that headache, twenty minutes

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Speaker 1: into the movie, you may notice that your eyes don't

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Speaker 1: feel quite right. It could be anything from excessive tearing

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Speaker 1: to an achiness that makes you take off the glasses

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Speaker 1: and rub your eyes every ten minute. It's have you

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Speaker 1: ever wondered what causes this effect? A letter from Walter Merch,

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Speaker 1: who was a famous film editor, to Roger Ebert tries

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Speaker 1: to get its arms around what causes this problem. He

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Speaker 1: calls it a convergence focus issue. You can understand the

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Speaker 1: problem in the following way. Hold your hand about three

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Speaker 1: inches away from your nose, and look at it. Two

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Speaker 1: things are happening. First, the muscles that control your eyeballs

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Speaker 1: are rotating your eyeballs, so your pupils are pointing towards

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Speaker 1: your nose. If you were to look at someone who's

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Speaker 1: looking at his or her hand three inches away from

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Speaker 1: his or her face, the person would look cross eyed.

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Speaker 1: The point of convergence is the distance of the object

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Speaker 1: away from your face. Now, when you're holding your hand

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Speaker 1: three inches away from your face, you're exaggerating this effect,

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Speaker 1: but the point of convergence is happening no matter what

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Speaker 1: you're looking at. Second, the muscles that control the lenses

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Speaker 1: in your eyes are manipulating the lens is so that

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Speaker 1: you can focus on something so close to your face. Again,

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Speaker 1: it's exaggerated here, but the focus distance has to do

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Speaker 1: with how far away an object is when you look

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Speaker 1: at it. This is completely normal any time you look

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Speaker 1: at anything in the real world. The muscles controlling the

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Speaker 1: convergence point of your eyeballs and the muscles controlling the

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Speaker 1: lenses are working in harmony. The two distances are the same.

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Speaker 1: In a three D movie, however, you have to do

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Speaker 1: something different. The focal length is constant. Your eyes have

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Speaker 1: to stay focused on the plane of the screen or

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Speaker 1: on the television set screen. Meanwhile, you have to change

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Speaker 1: the convergence angle constantly to get the three D effect

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Speaker 1: to work. This is not the way the system expects

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Speaker 1: the world to be, and it takes extra effort on

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Speaker 1: the part of your brain to make this happen. The

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Speaker 1: letter from Walter Merch points out that there's another side

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Speaker 1: effect to this problem besides the headaches that many people get.

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Speaker 1: It takes your eyes and your brain some amount of

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Speaker 1: time to figure out what the right convergence angle is

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Speaker 1: for the effect of each scene. So some scenes are

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Speaker 1: wanting you to see a three D effect that's nearer

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Speaker 1: your face, some want you to see a three D

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Speaker 1: effect that's far away, and so on. Your brain because

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Speaker 1: this is a little weird, is taking time to get

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Speaker 1: the convergence angle right for each different scene. This means

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Speaker 1: that three D films affect the film editing art. Scenes

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Speaker 1: can't be cut together as quickly as they can in

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Speaker 1: a two D movie, or they would confuse your brain

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Speaker 1: for moralns and thousands of other topics. Is that how

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