The demands for critically evaluating the color and sharpness of a transparency on a light box and the demands for telling if an image is sharply focused on a ground glass differ.
For use on a ground glass, mechanical properties are more important than ultimate sharpness and color correctness. For instance, does the loupe fit into the corners, does it keep your face far enough from the ground glass to minimize fogging in cold weather, does the focusing helical allow good focus on the ground side of the glass, does the base prevent scratching a the screen, which is especially important if you have a plastic screen, is it easy to clean, does it's magnification work well with the granularity of the glass?
I have an expensive Rodenstock loupe. It's terrific for looking at slide on a lightbox, but it isn't any better for ground glass viewing than a number of my other loupes.
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