Quote Originally Posted by Joe Symchyshyn
For an inexpensive loupe that has great quality, take your 50mm lens off the camera turn it upside down and use it
Joe, this is what I've been doing so far.
But it feels awkward to do so after a while, and I would like to be able to see the whole negative (or most of it) at the same time.

Since I am a bit on a budget, on some catalogue I have found a few very cheap "general purpose" loupes (I don't think they are strictly targeted to the photographer, but more to somebody who works on tiny objects like clocks, etc... ) And they are available at those magnifications that I have mentioned before.

From your contributions, 4X seems to be a good starting point, while I guess that for a 10X the optical quality should be more critical.