I'm not sure what problem a fancy grain focussing loupe is supposed to solve. I use a "cheap" Paterson Micro Focus Finder that is adjusted to my eye and reliably supports sharp grain in my enlargements. The Paterson focusser does not work in the corners of the image but it doesn't need to. My negative carriers have glass to keep negatives flat and I get sharp grain all over. If I suddenly started to get blurry corners changing the grain magnifier wouldn't help.
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