I've used this type of finder (Nicca, Tewe) on FSU cameras. Framing can be somewhat dicey. The location of the shoe that receives the finder is diagonally offset from the horizontal axis of the lens, so you won't be looking at quite the same view necessarily that the lens is. The higher the finder sits up off the top deck, and the longer the lens, the more pronounced this effect will be.
Both of the aforementioned finders have parallax correction gizmos that I personally find to be fiddly, inaccurate, and often overlooked (in my case) in the field. They provide yet another thing to adjust, if you're into that.
Of course, one can always make a difficult situation worse. I took my Tewe apart to clean it and forget to notice which way the curved side of the lenses faced, and when I finally did get eveything jiggered around right, the framing mask would inevitably slip out of place whilst tightening everything up. :bang:
So I advocate either the turret type finders, which by the way feature horizontal and vertical reference lines, or a nice bright dedicated viewfinder paired with your lens.
Cheers!
Peter
