Focusing on the GG: am I missing some optical principle?

[Ulophot]

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.)

[xkaes]

I can't think of any explanation. It sounds like you are doing everything correct.

Do you have the same problem with the adjustable loupe?

I might try it tomorrow with my adjustable Wista 7X loupe.

What powers are yours?

[Vaughn]

My guess is that you are re-focusing with your eyes to compensate for the difference. Put tape on the non-ground side -- see if you can focus your eye on the ground side of the GG and then focus your eye on the edge of the tape.

[Havoc]

I have no idea of the f-value of loupes but I guess that the same DOF applies to them as to lenses?

[Ulophot]

Thanks, all. I tried putting tape on the closest surface to me, and It seems my eyes are refocussing, although the action is unnoticeable. I can see sharply both the grain of the ground surface and the tape edge. I asked Alan Ross today (one of his free Zoom calls), and he had no explanation, either. Bit as long as the ground surface is in focus and it lies properly in the film plane, all is well. Mysteries of the Universe #7,934.

[Vaughn]

Well, there is another lens/focusing mechanism in the system that is not part of the loupe, but must be taken into consideration in its design.

Just got a Chamonix/Jobo 5x loupe. Not yet adaquately field-tested yet. First impression is that it is pretty sweet. I have not used one (a 4x) since the early 90s (being very near-sighted has worked for me).

[Doremus Scudder]

Thanks, all. I tried putting tape on the closest surface to me, and It seems my eyes are refocussing, although the action is unnoticeable. I can see sharply both the grain of the ground surface and the tape edge. I asked Alan Ross today (one of his free Zoom calls), and he had no explanation, either. Bit as long as the ground surface is in focus and it lies properly in the film plane, all is well. Mysteries of the Universe #7,934.

So, despite having your loupe focused on the frosted side of the ground glass, your eye can compensate well enough to focus significantly closer than that (and, presumably farther away as well) without adjusting the loupe?

Don't tell Bob Salomon...

Doremus

[Bob Salomon]

So, despite having your loupe focused on the frosted side of the ground glass, your eye can compensate well enough to focus significantly closer than that (and, presumably farther away as well) without adjusting the loupe?

Don't tell Bob Salomon...

Doremus

Why not?

[xkaes]

So, despite having your loupe focused on the frosted side of the ground glass, your eye can compensate well enough to focus significantly closer than that (and, presumably farther away as well) without adjusting the loupe?

Don't tell Bob Salomon...

Doremus

Exactly. The "problem" is that you are running a test with a non-adjustable loupe. If you try your adjustable loupe you might notice a difference.

I compared my non-adjustable 8X Agfa loupe and my adjustable 7X Wista loupe.

The Agfa assumes that my eyesight is fine (corrected with glasses if needed). So it works under the assumption that my eyes will adjust -- and see whatever is under it OK.

The Wista, on the other hand allows me to fine-tune the distance, as I see fit.

I suspect, if you use set your adjustable loupe for perfect focus on the ground glass, and then check a piece of paper on a light box, that a LITTLE adjustment of the loupe will provide a SLIGHTLY sharper image.

It did for me.

[Ulophot]

xkaes, it will have to be a very little indeed, but perhaps my eyes at 70 years and counting are a factor. I use astigmatism-corrected glasses (not severe astigmatism or diopter), and sometimes either very slight eye dryness or floaters require clearing a bit before getting a clear focus. In any case, as I said, I'm getting a sharp image where I place the focal plane with a wide open lens, so all's well.

This all started with a test to determine whether a 0.108 CoC is adequate for my work or if I should decrease it to 0.66 as per the model on the home page. Still ongoing.