Nintendo's 3DS, their new handheld console, can show images with depth—it's real 3D. But because you don't have to wear glasses to play, you have to hold the 3DS somewhat precisely. But how precisely?

Turns out the distance you hold the 3DS from your eyes matters very little. On a table? At arm's length? Nearly at the end of your nose? No problem.

But the tricky part comes with side-to-side movement. It takes very little off-axis movement to throw the 3D depth effect out of whack. That's not a big deal for traditional games where you simply hold the 3DS and push buttons. But for games that involve the motion sensor—which includes some of our very favorite Augmented Reality modes—it's very easy to move it too far.

Just watch the video. When the screen is green, our lovely, young model is reporting an acceptable 3D depth effect. When it goes to red it's out of whack.

[Update] According to @rjmatt, "the industry term for autostereo 'depth zone' is 'head box'."