So the KPS UMC arrived today, and I'm very happy with it! It fits on my Nikon D5200, though almost blocking the screen (it takes a little bit of practice or force to get the screen out, but it isn't in the way of viewing for me, and it isn't a huge issue), and the advantages outweigh this a lot.
There is of course a bit of vignetting. This is minimal. You have to get your eye pretty close to the viewfinder to see the whole image, but not any worse than the default viewfinder, actually a bit better I think. I can see the whole image, plus the info below, on my D5200 (1.5 crop sensor). I use the new viewfinder in a way where i press my eye socket to bend back the eyecup, which makes the eyecup effectively cover my whole eye socket. I don't see a significant drop in brightness.
The eyecup is great at blocking out everything else, especially good for night photography, where you want the screen to be black while looking into the sensor, as not to blind yourself in the lower part of you vision. An eyecup like this is great in general. It blocks out everything else
The actual magnification is good. I see no distortion or aberration. 1.3x is enough for me to be able to manually focus without issue, so it serves its purpose. My advice to anybody who like manual lenses, or want to enlarge the picture for some reason, these types of magnifiers are pretty damn great, at least the KPS UMC is!
So get one of these! Or the tenpa equivalent, that one should be pretty good too. But get one of them if you use a crop sensor dslr, it makes focus and composition so much easier!
Feel free to ask about it
Dear god, I sound like I was sponsored!