I'm visually impaired, and describing my way of seeing the world is very difficult
Going by your description, I don't really have a "natural FOV". My left eye is almost blind (7% sight only), so my right eye compensates for everything my left doesn't or can't do. According to doctors, I've got an extreme field of vision in my right eye, much wider than normal, and see "left and right" alternately to be able to see depth, using one eye to see both images normally seen by each eye seperately. I don't know how this works; my brain arranges for this compensation, it's been like this since birth
Depending on how I use my right eye (looking straight forward, looking out of the left or right corner, squinting, closing my left eye, which causes my right to move to the left to compensate), my FOV changes from extremely wide to very narrow. I can even block out all compensations my brain created during my life, which creates a view that I can't even describe; but it's very wide, and very, very flat.
Edit: I've just tested this for you. If I do not do anything special and look straight ahead, then my FOV is a little bit wider than my 15mm (effectively 22.5mm) on the GXR-M. It seems my natural FOV would be 21mm or so. Maybe because it's so close to my own vision is the reason why I like the 15mm so much. So not everybody "sees" in 50mm...
Told you above I had an extremely wide view

But my vision impairment prohibits me from seeing clearly very far off. Therefore I have "seen" many animals in zoos only through my 670mm DSLR setup. Without a 300mm x 1.4xTC x 1.6 crop factor, I wouldn't even have been able to see any of the animals. I've often used my DSLR as a "picture shooting monocular"...