Using a Starrett 449 depth micrometer on both the holder with a sheet of film and the groundglass frame with out the glass.
This is not a matter of using the DOF do cover the focusing error but just being as precise as I can. It really is surprising where the film lies vs the ground glass ground surface on all old holders. Why not fix it? I've been using these holders for decades and have complete confidence in my matchs. This is also very important when fitting a two part Frensel system. Think about that one.



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