Field of View and lenses.... in detail
Actually, 35mm film is called that because the film is actually 35mm tall (or wide when used vertically as movie film), including the sprocket holes and edges. In still photography, the 35mm film strip is run horizontally, and the recorded image spans the 24mm height in between the sprocket holes, and is 36mm wide. The diagonal measure of this frame is 43mm, or close enough to the standard focal length of 50mm. Thus a 50mm lens is considered "normal" for that film image size, yielding aproximately a 1x optical magnification.
So you can see how another post here is correct that what constitutes a "normal" lens depends on the film image size, measured diagonally.
Because we have two eyes with mostly (but not completely) overlapping fields of view, the ability to move our eyes, an uneven distribution of rod and cone cells (that detect color and brightness), and a brain that percieves details differently throughout our visual field, it's really tough to equate any lens to human vision. Our eyes, however, do not change focal length. We can focus on things near ot far, but focal length refers to the ability to "zoom" or change the field of view. We have a fixed field of view, but by concentrating on one detail or another, we can *percieve* differing fields of view.
Incidentally, each human eye has about a a 160 degree wide by 135 degree high field of view, biased slightly down and towards the outside, yielding an effective combined FoV of 200 wide by 135 high. About 120 degrees of the field overlap the other eye to facilitate binocular depth perception. Overall, 200:135 is almost exactly the same aspect ratio of standard "35mm" still photos at 36mm by 24mm, or a 3:2 ratio.
Chris Rakoczy
Rakoczy Photography
http://www.rakoczyphoto.com
close to the
The equivalence is somewhat based on the focal length versus imaging plane, i.e. 50 mm is about equal to the width of a 35mm frame, as someone mentioned already - hence the enlargement/reduction of objects is near unity, or 1x in optical terms.