I think that "naturally" and "pretty close" in that method are carrying a lot of weight. It is easy enough to hold up an SLR with a 50mm and see that the size

I think that "naturally" and "pretty close" in that method are carrying a lot of weight. It is easy enough to hold up an SLR with a 50mm and see that the size of an object in the viewfinder is essentially equal to what you see with your eye. It's almost like just looking through a piece of flat glass (apart from the slightly shift due to the lens being positioned below the viewfinder.)