On Tue, 02 Oct 2018 08:40:34 -0400, nospam
wrote:

In article , Eric Stevens
wrote:


the physical properties of a lens are *not* an opinion.

But what is normal and hence what is long is an opinion.

no

Yes. I have already given you the Wikipedia definition of normal
https://en.wikipedia.org/wiki/Normal_lens "... a normal lens is a
lens that reproduces a field of view that appears "natural" to a human
observer."

Under the heading of "What is 'normal" it says

"A test of what is a normal lens then, is to find one that renders a
printed (or otherwise displayed) photograph of a scene that when
held at 'normal' viewing distance (usually arms-length) in front of
the original scene and viewed with one eye, matches the real-world
and the rendered perspective, though Maurice Pirenne (in 1970) and
others demonstrate that it is possible to see a scene made with any
lens in normal perspective if one adjusts the viewing distance; but
impractically close to the image for wide angle photographs and
further away for a telephoto, and views it from a static point at
the centre of perspective from which the image was made ... "

In other words that while whether or not a lens is normal may be a
subjective judgement the work of Maurice Pirenne shows that any lens
may be regarded as normal.

Lower down the Wikipedia article says

"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; its angle of view is similar to the angle subtended
by a large-enough print viewed at a typical viewing distance equal
to the print diagonal;[12] this angle of view is about 53°
diagonally. For cinematography, where the image is larger relative
to viewing distance, a wider lens with a focal length of roughly a
quarter of the film or sensor diagonal is considered 'normal'. The
term normal lens can also be used as a synonym for rectilinear
lens. This is a completely different use of the term."

It is clear that even for constant image dimensions the focal length
ascribed to a 'normal' lens is very much a moveable feast dependent on
the viewer and their circumstances. That is why I wrote "what is long
is an opinion".
--

Regards,

Eric Stevens