Originally Posted by andephotographic
I think what you are saying is that unless the viewfinder was made specifically for a particular camera the view through it won't exactly match what's on the neg (or sensor) right?
Well, the finder can only be accurate to specification for cameras with a hot/coldshoe position (relative to the lens) identical to the one it was made/adjusted for.
But given that parallax can only be compensated for flat field subjects in one single distance, specifications are far from general, and it might still be accurate at some other distance - and parallax corrections only work within sometimes impractical constraints in any case. Even the best parallax-compensated brightline rangefinders are only a very coarse framing guide once you get to close-up work in a three-dimensional world - if you need high precision framing, there is no way around a (SLR/view camera/live electronic) finder using the image from the main camera lens.
It is trivial to test whether a given auxiliary finder will work for you - move the finder by the difference of hot shoe positions between the original and your camera, and it will become evident whether and for which distances it is relevant...