The Schneider 3x Loupe is superb and covers up to 6x7 with no distortion at all.
Yes, exactly, this loupe is excellent. Very, very sharp and contrasty, no chromatic aberrations, no distortion. The loupe covers a field of 8x8 cm, so you can even look at 6x8 slides from some Fuji MF cameras without problems.
I am using it even for my 6x9 slides, you loose only 0,5 cm at the ends, and that is not a problem with most of my 6x9 slides.
The Schneider loupes are shipped with a transparent foot, too. So they are even suitable for looking at prints.
I can highly recommend the Schneider 3x loupe (and the 4x for 35mm, too). It is a joy to use.
Best regards,
Jana