Here is my take on this issue:
The definition of a "normal" lens is purely arbitrary (convention) with some
(little) basis on the physiology of vision.
With any lens used, it is usually possible (with maybe the exception of
ultra wide lenses) to view a picture so that the "viewing distance" matches
the "recording distance". Let's call this "ortho-viewing" (ortho =
correct).
For example, using 35mm film take a picture with a 28mm or 50mm or 135mm
lens and then view the film directly using a lens with 28mm, 50mm or 135mm
focal length, so matching the focal length of the recording lens with the
focal length of the viewing lens. (Instead of viewing the film directly,
you can view an enlarged print at a proportionally enlarged/longer viewing
distance.)
With ortho-viewing you get the correct perspective. If you do not view
under this ortho condition, you will experience perspective distortions.
The so-called wide angle and telephoto perspective distortions are simply
the result of viewing the picture under non-ortho conditions (wide angle
photos viewed from a longer distance and telephoto photos viewed from a
shorter distance).
So given that for any FL lens we can have ortho viewing conditions (with the
field of viewing changing going from wide to narrow), where do we draw the
line between wide and long and call it "normal"?
Traditionally this line is drawn at the diagonal of the image size. This
choice is mostly arbitrary, a matter of convention.
The physiological basis for this choice is that when we look at a picture,
we tend to place ourselves at a distance equal to the diagonal of the
picture. I have done this experiment (anyone can do it): I took pictures of
different sizes from a stamp to large paintings/computer TV monitors, and
moved myself back and forth until I found what I thought that it was the
most comfortable viewing distance. Then I measured this distance and
compared it to the picture diagonal. In most cases, I was standing further
than the diagonal. Also, the viewing distance was a function of the size of
the picture (I forget which way it went).
The bottom line is that drawing a sharp line to define the "normal lens"
does not have a basis in science. The line is not sharp and the definition
is a matter of convention. The choice of the word "normal" is a bit
unfortunate too.
George
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