The VIOOH (Imarect) is fun. The image does get rather small at 90mm, and ridiculously small at 135mm. It has very accurate framing, due to the complicated design that puts both the viewed image and the frame in sharp focus.
The VIDOM does the same thing, but with a less sophisticated prism design that leaves the image swapped side to side.
Both are the type of telescope that leaves the image completely flipped (just like a lens on a camera), so the prism systems correct for that. They also make the optical path physically longer, otherwise they would need to be very long indeed.
Note that the VIDOM is longer, since the roof prism it uses doesn't fold the optical path as much.
A nice thing about the VIOOH is that the reflecting surfaces of the prisms aren't slivered, they're just flat glass. The non-reflecting surfaces are painted black. Thus, you don't need to worry about failed silvering, you just clean all the exposed glass surfaces. Of course, if the prisms have come unglued, it's "game over" time.
Both of these finders make the image more like a "picture" in a black mat, they can help you think about composition. (This is why HCB liked the VIDOM.) But, they leave you with no peripheral vision, not good for action like a bright-line finder.
All the zoom finders seem to be of the same basic design. I have the Walz one, maybe they copied the German Tewe one? They have the advantage that the magnification goes up as you zoom towards telephoto, so the image is only a little smaller at 135mm. But they have disadvantages. The frame mask is not in focus, so you have a soft edge like any Galilean finder. There's barrel distortion at 35mm. Finally, the diopter gets rather strong at 135mm, maybe +1, and I just can't focus my eye enough past infinity to use it out there. (Maybe I could have when I was 20, but not at 48.) I don't know if all the zoom finders share this last trait, but know they share the others.
