Humans have two eyes, which is what allows us to see 3D in real life. Our eyes are 2-3 inches apart and both facing the same direction. So filming in 3D requires using two cameras with the lenses 2-3 inches apart recording simultaneously. Then, one of the various 3D technologies is used to separate the resulting video and show each of your eyes a different image.
The simplest of these is the age-old red/blue pair of glasses. One eye only sees the red image, and one eye only sees blue, producing the 3D effect. For big budget movies in the theater, a more advanced type of 3D is used called circular polarized. This produces a full color 3D effect, but requires a special screen and a very expensive projector.
Anyway, to make your own 3D home video, all you need is a pair of cameras, and basic video editing skills. First, find a way to mount your two cameras so they stay perfectly in line and level with each other. This article details how to build a cheap mount for two cameras. I built one using two 60¢ wooden yardsticks from Home Depot.
Once you have your cameras mounted together, simply start recording with both cameras, and have fun recording your video. After you're finished recording, import the video into your editing software as always, being sure to label which video came from the right and left cameras.
Now for the tricky part. First, you have to sync the two videos so they're playing perfectly together. Then, combine the two video sources into a single frame, distorting them sideways to do so. The video from the right camera goes on the left, and the video from the left camera goes on the right:
|(Taken from YouTube's 3D help page)
So what you end up with is a a single frame with two vertically squished videos on either side. It will look pretty strange in your video editor, but the final product should look like this:
|(additionally, if you cross your eyes a little, the image above will appear 3D)
Then you simply upload your video to YouTube as normal, and add yt3d:enable=true to the tags. Make sure you put that in the tags, not the title. (Also, if you mix up your left and right videos, you can add the tag yt3d:swap=true and it will swap your right and left videos for you. The enable tag tells YouTube that your content is 3D ready and it does all the work of combining the videos and giving you the drop down for selecting which type of 3D glasses you have, etc.
And that's all there is to it! The hardest part is building a mount for your two cameras. Once you have that, it's basic editing and uploading, just with a few extra steps. If you have access to two identical (or at least similar) cameras, I highly recommend giving it a try. It's a really simple way to create a very impressive effect. I've only made two 3D videos so far, but more are in the works, so keep checking my 3D playlist for a list of the videos I've made.
3D is definitely here to stay and I fully intend to continue perfecting my technique. If anyone wants more help or advice on making 3D videos for YouTube, feel free to ask, I'd be happy to help.