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