Originally posted by tvdtvdtvd The 43mm corresponds to the diagonal across the 35mm/FF frame, that much I remember. I forget the justifications for the other two, though from my own personal experiences the 77mm is MUCH a easier walk around FF than 85mm.
The 43mm is almost exactly the angle of the human eye view. The 3:2 ratio of the first film format is anything but random too. No wonder maths agree with that. I would really love to know about the other two lengths, they are peculiarly unrelated...