The extra viewfinder has nothing to do with it. It is the ability of the range finder...the big window with the blue/yellow tint and the little window on the other side of the distinguish a difference in what they see that makes the difference. The further apart they are, the more they are able to see something slightly differently and therefore focus. Even with the Mamiya's 150mm lens they can barely do that, which is why there are so many complaints about that lens's ability to focus and why people tell you to ignore the hyperfocal marks on the barrel.