So, to answer some questions: Yes, I did calibrate the grain focusers to my eyesight. The Micro Sight and Micromega Critical focusers make it relatively easy to do. So that's taken care of.
I didn't use a piece of paper under the focuser. But that was consistent. I didn't put paper under *any* of the focusers. It doesn't explain why the results from the four instruments would differ.
For Gerald Koch: You say that if I stop down, there shouldn't be a problem. I respect that, but it still bothers me. Surely the grain is either in focus or it isn't. I do have a couple of negatives that I have to print wide open, particularly if I'm doing a large enlargement. When I'm lith printing, I routinely print wide open.
Is there any way to calibrate a grain focuser other than by adjusting it for your eyesight? I suppose there really isn't.