The ground surface of the ground glass faces the lens and the glass is several mm thick. When I use my focusing loupe to focus the picture, am I focusing on the ground surface of the glass or some plane within the glass or on the un-ground surface? If I have a fresnel lens, what plane am I focusing on?
The image should be formed on the ground surface, so you should focus your loupe to on the ground glass. What the ground surface does is scatter the light rays -- if the camera lens has focused an image onto the ground surface, the loupe will intercept the rays scattered by the ground surface and bring them into focus for your eye. If the image isn't in focus on the ground surface, the loupe will focus the scattered rays into an image for your eye, but the original image won't have been sharp, so the loupe won't make a sharp image for your eye.
It is helpful to start with a very blury image on the ground glass, even no image, just shinning a light on the glass without using a camera lens, and then to focus your loupe on the grain of the ground glass.
The ground glass could be any any thickness, as long as the ground surface faces the camera lens and the glass is thin enough that the loupe can focus on the ground surface.
Without a fresnel lens, the ground surface should be the same distance from the lens as the film. If the fresnel is placed behind the ground glass (the side away from the camera lens), this is still true. What the fresnel lens does is direct more of the rays scattered by the ground glass towards your eye or loupe. The image should still be formed on the ground surface.
If the fresnel lens is placed in front of the ground glass, between the camera lens and the ground glass, the situation is more complicated. The image should still be formed on the ground surface, but the fresnel lens, because its index of refraction differs from air (which has an index of refraction of almost exactly one), will cause the effective optical distance between the lens and ground glass to differ from the physical distance. To maintain correct focusing, the film plane and ground surface of the ground glass need to be at different distances from the lens. My opinion is that this is a poor design choice.
Michael has pretty much said it all. The only thing I would add is that under very high magnification it is possible to focus instead on imperfections in the glass or the Fresnel ridges, and that can lead to focusing errors. It is for that reason that one is sometimes advised not to routinely use a loupe of greater strength than about 4 X. I have a Maxwell screen, and I get very precise focusing at about 4 X, but when I use a 7 X loupe, I can see the Fresnel ridges, which are very fine. I can still see "through" them to the image, but it requires concentration.
Greetings,
I seem to recall seeing/reading somewhere that a focusing loupe needed to be tweaked for sharpness. Of course, this is for those loupes that have the capability of being focused.
It had something to do with removing the lens and shining a light on the groundglass... or something like that.
Michael, Leonard... does this make any sense at all to either of you? Or, do I need another glass of wine? It's been a very loooong day.
Thank you.
Life in the fast lane!
Micheal,
My apologies... you've answered my question. It is focusing on the groundglass when a light is shone onto it. I should have read your posting more carefully.
Thank you.
Life in the fast lane!
Thanks, Michael, for the most succinct explanation of this topic that has appeared on any of these large format forums for some time. This is an archive keeper.
Best regards,
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