Originally posted by reh321 on a Q7, it gives roughly the same field of view that a 40mm lens would give on a 35mm film camera.
Of course, 50mm was considered to be the standard lens in 35mm film days.
The designation of a normal or standard lens is somewhat arbitrary, since you never know how the image is going to be viewed. However, according to Wikipedia: "For still photography, a lens with a focal length about equal to the diagonal size of the film or sensor format is considered to be a normal lens..." If you follow that guidance, then a "true normal" lens for a full-frame 135 format camera would be 43mm. (And I don't think it's coincidence that we have the Pentax-FA 43mm Limited.)
35mm film cameras are sort of the exception to the rule. The original Leica shipped with a 50mm lens -- according to legend, because that's what Oskar Barnack had handy in the workshop when making the prototype in 1913! The Leica was, of course, highly influential and widely copied. And so the nifty fifty tradition began.
However... It seems like most medium format cameras are closer to the "true normal" film diagonal, though. The diagonal of 6X6 format film is 79.2mm, and my Ricoh Diacord G has an 80mm lens. The diagonal of 6X9 film is 101.0mm, and my Fuji GW690 has a 100mm lens. They stayed very close to the formula.