Magnabrite Loupe?

Has anyone tried those acrylic "light gathering" magnifiers called Magnabrite:

http://www.saundersphoto.com/html/magna.htm

for ground glass focusing?

The problem with using the magnabrite as a focusing loupe is that the are magnif ying the rear surface of the ground glass / fresnel combination and not the imag e formed on the frontside which is where the film plane is.

They are great hands free contact sheet magnifiers though.

Ellis makes a good point. A ground glass loupe really needs to be focusable, so while sitting on the back of the glass, it can be adjusted to focus on the front surface where the image is formed. For looking at contact sheets, especially 35mm; I've found that a good quality 'bifocal' hand held magnifying glass works really well. The lower power overall magnification allows the client to see a magnified whole image..and the little, higher power spot allows them to look at details at a much higher magnification once they bring the glass into the correct focusing distance

I tried a glass version of the magnabrite sold by Brookstone and as expected it doesn't work on ground-glass and transparencies. It is also prone to scratching prints for which it does a "reasonable" job.