3--A "Normal" Lens is 50mm (32mm in DX-dom)
One recent photography book I was browsing through repeated the oft-made claim that a 50mm lens most closely matches that which our eyes see, the so-called "normal" lens. The problem with this, of course, is that the eye/brain connection includes a number of subtle features, such as peripheral vision and near-constant reorienting and focusing. In general, I find that I "see" about a 24mm-equivalent field of view, with my vision concentrated on the equivalent of anything from a 80mm to 300mm lens (and this range has narrowed as I grow older). (For those that are curious, most human eyes are about 16mm in focal length and the pupil's iris can manage effective apertures of from about f/2 to f/11.)
What most texts are referring to when they anoint "normal" lenses is that the focal length is approximately the diagonal of the image format. That's it, there's no hidden meaning in that definition, and almost no human connection. One reason why such lenses are useful, however, is that the format diagonal tends to be the focal length that allows large usable apertures coupled with minimum performance compromises. As you make shorter or longer focal length lenses for a format, distortion, coma, chromatic aberration, and field curvature become more and more difficult to control, especially if you want to retain large apertures.
http://www.bythom.com/myths.htm
