One of the best loupes I have is actually a finger print analyzer that I got out of an estate that I liquidated for a law professor, sharp, no distortion and about 8x, really allows you to see sharpness and I also use for my fine focus on large format, they normlly run about 10 bucks now if you can find one, but is beats my $200.00 Rodenstock, I love it.

Dave Parker
Ground Glass Specialties