The Focal Length of the Human Eye
I learned a really interesting fact today: The focal length of the human eye is equivallent to 17mm ~22mm. That means that a photo taken with a standard lens less than 17mm ~22mm will appear distorted compared to your normal vision. There are exceptions, of course. Most wide angles, despite their focal length, will also distort the image because of its angle of view.
Remember to take your camera's crop factor into account. Those with a Nikon DX format camera (D40, D60, D80, D90, etc) have a crop factor of 1.5 which means your 50mm lens acts like a 75mm lens and so on.
For more reading about the human eye's optical characteristics, check out the Notes on the Resolution and Other Details of the Human Eye over at Clarkvision Photography.
Edit – further research seems to lead me to believe that the focal length of the human eye is closer to ~22mm. The 17mm is actually based on studies of light coming out of the eye (the inverse)…as if you were to turn your lens around backwards on the camera. Of course the more I learn about this, the more I think that this isn't such an exact number to pin down. I'm going to continue my research (mostly because I'm curious) and I'll continue to post my findings as they come up.