Quote Originally Posted by modafoto
How do you calculate it?
How do I calculate what?

If you are referring to the human vision part (the agle of view), I don't calculate it. I "copy" it from books/texts/etc. I beleieve there is a (more or less) commonly accepted way of calculating it taking into account the degree of sharpness in the complete human fov(?) (which is about 180 degrees horizontal and 90 or so vertical)

If you refer to film it is the diagonal of the 35mm frame.