I've noticed that in terms of real-world magnification, cameras vary in viewfinder image size even when the same lens is mounted. On some of my 35mm cameras, with a 50mm lens the image in the viewfinder is very nearly life-size. In this case it's pretty comfortable to open both eyes and then you can superimpose the viewfinder on the world with both eyes open and it's pretty liberating. It's never exactly life size though, there is usually a nagging few percent difference.

Do you know of a camera+lens combination that gives exactly 1x magnification with the real world, so that looking through the viewfinder is like looking through a window?