"Paul J Gans" wrote in message
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[ . . . ]

I dimly recall the term "prime lens" as being the
lens whose focal length was equal (at least roughly)
to the diagonal of the film frame. That made a 50mm
(or 45mm) lens "prime" for 35mm film.

I have never seen "prime" used in that way, but at least it's no more
incorrect than the way it's mostly being used in these newsgroups. ;-)

It used to be considered that the proper (or normal) focal length for a
camera was the diagonal of its negative. Similarly, the length + width of
the negative was considered a suitable focal length for portraits. I don't
think anyone has paid much attention to those rules of thumb since we've had
such a vast range of focal lengths available to us which were undreamt of a
few decades ago.

N.