WEBVTT - Why do 3D movies cause headaches?

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<v Speaker 1>Welcome to brain Stuff from house stuffworks dot com where

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

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<v Speaker 1>do three D movies and three D televisions give lots

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<v Speaker 1>of people headaches? The vast majority of people can go

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<v Speaker 1>watch a normal two D movie, or can lie on

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<v Speaker 1>their couch and watch their normal two D television set

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<v Speaker 1>without any major problems. They can watch for hours, and

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<v Speaker 1>in general, the vast majority of people don't get a

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<v Speaker 1>headache from doing that. But three D movies are different.

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<v Speaker 1>Three D movies cause a lot of people to get headaches.

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<v Speaker 1>And even if you don't get that headache, twenty minutes

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<v Speaker 1>into the movie, you may notice that your eyes don't

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<v Speaker 1>feel quite right. It could be anything from excessive tearing

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<v Speaker 1>to an achiness that makes you take off the glasses

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<v Speaker 1>and rub your eyes every ten minute. It's have you

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<v Speaker 1>ever wondered what causes this effect? A letter from Walter Merch,

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<v Speaker 1>who was a famous film editor, to Roger Ebert tries

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<v Speaker 1>to get its arms around what causes this problem. He

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<v Speaker 1>calls it a convergence focus issue. You can understand the

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<v Speaker 1>problem in the following way. Hold your hand about three

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<v Speaker 1>inches away from your nose, and look at it. Two

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<v Speaker 1>things are happening. First, the muscles that control your eyeballs

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<v Speaker 1>are rotating your eyeballs, so your pupils are pointing towards

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<v Speaker 1>your nose. If you were to look at someone who's

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<v Speaker 1>looking at his or her hand three inches away from

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<v Speaker 1>his or her face, the person would look cross eyed.

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<v Speaker 1>The point of convergence is the distance of the object

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<v Speaker 1>away from your face. Now, when you're holding your hand

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<v Speaker 1>three inches away from your face, you're exaggerating this effect,

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<v Speaker 1>but the point of convergence is happening no matter what

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<v Speaker 1>you're looking at. Second, the muscles that control the lenses

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<v Speaker 1>in your eyes are manipulating the lens is so that

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<v Speaker 1>you can focus on something so close to your face. Again,

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<v Speaker 1>it's exaggerated here, but the focus distance has to do

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<v Speaker 1>with how far away an object is when you look

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<v Speaker 1>at it. This is completely normal any time you look

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<v Speaker 1>at anything in the real world. The muscles controlling the

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<v Speaker 1>convergence point of your eyeballs and the muscles controlling the

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<v Speaker 1>lenses are working in harmony. The two distances are the same.

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<v Speaker 1>In a three D movie, however, you have to do

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<v Speaker 1>something different. The focal length is constant. Your eyes have

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<v Speaker 1>to stay focused on the plane of the screen or

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<v Speaker 1>on the television set screen. Meanwhile, you have to change

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<v Speaker 1>the convergence angle constantly to get the three D effect

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<v Speaker 1>to work. This is not the way the system expects

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<v Speaker 1>the world to be, and it takes extra effort on

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<v Speaker 1>the part of your brain to make this happen. The

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<v Speaker 1>letter from Walter Merch points out that there's another side

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<v Speaker 1>effect to this problem besides the headaches that many people get.

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<v Speaker 1>It takes your eyes and your brain some amount of

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<v Speaker 1>time to figure out what the right convergence angle is

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<v Speaker 1>for the effect of each scene. So some scenes are

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<v Speaker 1>wanting you to see a three D effect that's nearer

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<v Speaker 1>your face, some want you to see a three D

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

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<v Speaker 1>this is a little weird, is taking time to get

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<v Speaker 1>the convergence angle right for each different scene. This means

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<v Speaker 1>that three D films affect the film editing art. Scenes

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<v Speaker 1>can't be cut together as quickly as they can in

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<v Speaker 1>a two D movie, or they would confuse your brain

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<v Speaker 1>for moralns and thousands of other topics. Is that how

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