Hello, I can't tell, but the problem is (with an 17mm FD lens, haven't yet tried with other lenses) that if i move/raise the finder's level, I assume this will do magnifications, everythings starts to get blurred no matter if I try to get back the focus via the lens ring. So basically I have to leave it low on the finder and profit from its luminosity. Is clear that one can better focus through it than through the standard finder that it's significantly smaller and some light could come in by the sides.
I've just developed a film and it looks like i got good focus on some shots. Will see this afternoon after scanning the film what the real results were.