## 3D again

A while ago, I wrote a bit about how 3D movies work.  One thing I said was

One experiment I wish I'd tried during te movie: put the glasses on upside down, so that the image meant for the left eye goes to the right eye and vice versa.  This should have two effects:

1. Make you look even goofier than the other people in the room wearing 3D glasses.
2. Show you the picture inverted in depth (close stuff looks far and far stuff looks close).

If you try this during a 3D movie, let me know if it works (particularly #2 €” I'm pretty confident about #1).

As every theoretical physicist knows, there’s a danger in making predictions: someone will go out and do the experiment.  My friend Tim Savage comments

Ted, we went to see the 3-D version of "Cloudy with a Chance of Meatballs" yesterday, so I tried your experiment of turning the glasses upside down. I'm sorry to report that you were wrong. It doesn't reverse the depth, it just flattens things out a bit. That is to say, the things that are supposed to be forward are still forward, just not as much so, although a little bit more than in a traditional 2-D picture. It does give you a headache if you try it several times, though.

A crushing blow.

Even stranger because I’ve subsequently read a bunch of things, including this article in The Physics Teacher, indicating that the model I described was more or less right.   I’m at a loss to explain the data at the moment.  I think I’ll have to go see another 3D movie to see if I can reproduce Tim’s result.

Any further experimental data or theoretical insight will be gladly welcomed.

### 4 Responses to “3D again”

1. Andy says:

I think you should carefully consider observer error.

2. Ted Bunn says:

That’s certainly the explanation most consistent with Eddington’s principle. (Never believe an experiment until it’s been confirmed by a theory.)

It’s worth bearing in mind that stereoscopic vision is only one of several cues used to perceive three-dimensionality, so it’s possible that even if I’m right about the physics my prediction was wrong as a matter of human perception.

I think I want to see Tim Burton’s Alice in Wonderland, so I’ll try the experiment myself then.

3. hiphop beats says:

shouldn’t it work if you turn your glasses around backward, not flip them upside down? i thought the newfangled 3D glasses use 45-degree reflections that send the image towards the center of your eye. upside down lenses would possibly send the image out to your peripheral vision.

4. Brent Follin says:

New experimental evidence:

Turning your glasses 180 degrees about the horizontal axis does invert the image, but because of the strength of other visual cues, it’s hardly noticeable unless you focus for a while on something in particular. The effect is more just to make things sort of fuzzy and flat–and also a bit confusing. Fast scenes (like the trailer for How To Train Your Dragon before Alice in Wonderland) are difficult for your brain to process–sorting out the cues becomes very confusing.