This has been one of those days of frustration that I thought was long in my rearview mirror, but here I am.
I have two Wista loupes, identical except that one focusses and one doesn’t. I have been using the non-focusing one, for which I long ago made a collar that extends the tube a bit for sharp focus on the ground surface of the GG, which I found necessary long ago.
What is bewildering me at the moment, is that I have focused the first one so that that ground surface of the GG is in sharpest focus, i.e., the “grain” of the surface, as confirmed by taping a piece of cellophane tape on that surface and seeing the edge tack sharp. However, when I place the same loupe directly on a sheet of printed paper on my light box, the printed type is in perfect focus. Surely, this should not be, because the ground surface of the ground glass faces the lens (wait, please, if you consider this wrong), and I have a thinner sheet of protective glass of some type that came with the camera, between the GG and the loupe/me. To be crystal clear, the progression is: back surface of the lens, bellows air, ground GG surface, thickness of GG to unground GG surface, and thickness of clear protection glass, which is a bit thinner than the GG glass, loupe.
It takes vary little displacement of the loupe to defocus whatever surface-distance it is set for. Therefore, why should the GG surface and the printed type both appear to be in sharp focus without changing the loupe’s focus, when the GG ground surface is farther away by the two thicknesses of glass? Is there some optical principle of aerial images I’m missing here?
Regarding the placement of the ground surface: I have read advice in this forum for a variety of configurations, all with justifying arguments. According to my measurements taken again today, the ground surface of the GG is exactly the same distance as the film is when the holder is inserted, and I reconfirmed this immediately afterward by making a full-aperture (6.3 on a 210) photo of a target designed to check my focus. So, when I focus on something through the camera with my loupe, the neg comes out sharp.
(The frustrating back-story as to why I went through all this today I will leave out. Sometimes I make errors I can’t believe.)
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