When I read:
A lens is considered normal, if its focal length equals the diagonal of the film format, which is ~43mm for 24x36. So 40mm has always been more normal than 50.
... my first thought was trying to work out in my mind how that rule would apply equally to a rectangler format like 135, and a square format like 6x6cm?
Then I read:
Using a 50mm lens on 35mm format camera is like using an 80mm lens on 6x4.5cm format. Both are the diagonal of a hypothetical square of the longest dimension.
In both cases it is a little oddball; I’m not a fan of either.
Interesting. Intuitively "the diagonal of a hypothetical square of the longest dimension" appeals to me - and it explains why many say 50mm is the normal for 135 film, and perhaps fewer say 40/43mm(?)
I am guessing it might not be possible to define the concept of a "normal focal length" using only the science of optics without introducing some subjective criteria? So the concept of a "normal" focal length lens may be too vague to defend, except as a matter of opinion and convenience (as opposed to conforming to some law of science)?
This discussion in an article in the
Atlantic magazine gives some historical perspective <
https://www.theatlantic.com/technology/archive/2018/05/how-the-50-mm-lens-became-normal/560276/> Warning it is the
Atlantic magazine - so expect some literary flourishes.
This article by Roger Cicala does a pretty good job of exploring why trying to describe any photographic lens as accurately corresponding to human vision is on shaky foundations: <
https://wordpress.lensrentals.com/blog/2009/03/the-camera-vs-the-eye/>
Of couse, accurate definitions, optical theory, and long standing traditions all take a back seat to what is really important - which is, what focal length gives you the results you like best? If I am carrying my bag for 135 film gear, it will have both 50mm and 35mm lenses, and they probably get about equal use. But if I want to carry a camera, only (no bag), then sometimes I will use only my compact 40mm.