The theory makes no sense. We have peripheral vision and our "natural " FOV is a lot wider than 40mm.
A second way to objectively judge "normal" is from the diagonal of the sensor. This results in 43mm being the normal for 35mm full frame, which is why Pentax created a 43mm lens. An excellent lens. 21.5mm would be the same for MFT, so the 20mm wins.
That's the thing I was getting at - there arent any lenses that see the world like the human eye, because human vision is more like a fisheye lens, with a 180 by 100degree, binocularly shaped field of view.
The theory of a focal length giving a natural perspective (which is not necessarily the same as mimicking human vision) can be correct, but ONLY if you relate it to the image size and viewing distance. It's exactly the same as when you're standing in front of a window looking out. The closer you are to the window, the wider an angle of the outside world you'll see, and conversely, the further you are, the narrower the angle of the outside world. For instance, if you're standing one meter away from a window that's two meters wide, your vision of the outside world will have a horizontal fov of 90 degrees.
If you then consider a photo as a window to the world, the principle is the same. If the photo is two meters wide and you're one meter away from it, an 18mm (equivalent) lens on a 3:2 ratio photo would give you the same perspective that you'd get if the photo wasn't a photo but a window of the same size, so it feels natural. If the photo was instead taken with a 36mm lens, giving half the horizontal fov, it'll feel too narrow; it's not a natural continuation of the 90 degree angle to the edges of the photo frame. This can then be remedied by stepping back one meter, making the angle to the edges of the photoframe an identical 45degrees to the field of view from the 36mm lens; thus once again making the photo seem like looking through a window.
As I indicated in my previous post, assuming a viewing distance identical to the image diagonal does indeed give a natural perspective with a 41.5mm lens, but it's completely arbitrary; one might as well consider a viewing distance of 20 times the diagonal is normal, making a 930mm lens "normal". As I indicated, in the printing business, 1.5 to 2 times the image diagonal is apparently considered normal; I assume that's because people prefer to view an image from roughly that distance. If any focal length were to be considered normal (because it gives a natural perspective at the preferred viewing distance), 60 to 80mm would therefore be the prime candidate.
But neither of these points matters. Because each of us has our preference for what we like to see. For me the 45mm is glued to my camera, for equivalent FOV 90mm. I love it.
Go with what you love and forget the justifications.
I think everyone agrees on that part.
