In article <1996Jan11.182838.8492@news.wrc.xerox.com>, mcfarlan@xerox.com says...

>Somehow I got it into my head that a "normal" lens
>was based on the length of the diagonal of the film
>area. For example, a 6x6 would be sqrt(36+36) = 8.5
>= 85mm lens.
>Is this way of determining the "normal" lens correct ?
>If not, how is it determined ?

The diagonal of the image area is the technical format normal
focal-length, but it is often quite different in practice. In
35mm, it works out to be 43-44mm, but almost everyone considers
50mm to be "normal". In 16mm film, 25mm is considered "normal",
but that is far longer than the diagonal of the image area.
So, I guess, the "normal" focal lengths for the different
formats are the ones that have come to be considered
"normal". (Notice that I said NOTHING about "the
way our eyes see", or anything like that!)
Hope This Helps