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# Thread: Placing the GG correctly

1. ## Placing the GG correctly

In designing my 20x24" camera I have to think about a lot of small things. One of the the corect plament of the GG. Does the GG frontside(towards the lens) have to be in the same spot as the film ? Or is it better to use the backside or middle ? Does it matter at all ?

2. My understanding is that the glass surface that the image forms on has to be in the same plane as the emulsion side of the film. It matters because if you focus your camera based on the image formed on your GG but then put your film somewhere else you won't get a focused image on the film. Presumably there's some tollerance based on the depth of focus, but I don't know how much...

3. That is my understading too, but wich side of the GG has to be the same as the emulsion ?

4. The image is formed on the ground/etched surface which is on the lens side of the glass (the outside surface is smooth on all my cameras).

5. Ian is 100% correct. Ground side of the glass should be facing the lens and its surface should be in the same position as the emulsion side of the film.
For a camera of 20x24" format, the tolerance is a bit wider than for the smaller formats.

Greetings,
G

6. If you were to unknowingly or erroneously (I've seen someone do it when reassembling a camera) put the ground glass in backward, ground toward your eye, and not toward the lens), you could still see and focus.

However, what happens with the thickness of the glass between the lens and the ground glass image is that the focal point is set further back...there is an equation for a flat glass element (some lenses include them, patent for Metrogon or maybe Topogon covers this, also discussed in photogrammetry books I unloaded onto someone else), with resultant focus shift IIRC = t/n, t being thickness of glass and n being index of refraction.

If n = were 1.5, f.p. is shifted back (longer) by 2/3 the glass thickness.

The nuisance is determining how far the film plane is offset from the camera back when a film holder is placed in the camera. The glass normally has to be spaced to match a film holder. The yahoo cameramakers group has a document for 2x3 thru I think 8x10 ANSI specs including tolerances for filmholder dimensions. As I understand it there are no consistent standards for film formats larger than 8x10 (or is is 11x14?), so different banquet size ULF cameras differ from brand to brand.

I imagine the people making/selling ULF filmholders have agreed upon some sort of standard (it may be conformity to a given popular camera's glass spacer).

I have been thinking about a glass film holder analogous to enlarger glass negative carriers, for a specific camera design...I can't imagine it costing as much as custom wooden ones, but they would be heavy and awkward.

1) Build camera with ground glass made with anti-reflective glass and spacer determined by step 2 below. Mount with ground side AWAY from lens (opposite of above discussion)

2) Glass film holder consists of a piece of anti-reflective glass at front hinged with regular glass at back. Film is loaded between AR and regular glass panels. AR glass supposedly serves as anti-newton glass (according to some glass-negative carrier page I read. By AR I mean sputtered metallic coated glass (like eyeglass AR coating), not blurry non-glare framing glass. The 2-pane glass-film sandwich would slide into a grove in a light tight simple wooden or aluminum 'holder'. The thickness of the front of the holder assembly and any darkslide would determine the thickness of any spacer required for the in-camera ground glass.

This would be heavy, awkward, maybe a bit impractical, but I think it would work, probably cost less and allow use of unconventional rollfilm like thin-base aerial film that is too floppy to stand up in conventional filmholders.

Seems like most people who use modern ULF cameras can afford the conventional film and filmholders, but I offer this idea for people as strange as me who like to do things the hard way.

7. If I'm not mistaken there are ANSI standards for 11x14 and 14x17 filmholders. These sizes were used in the medical field, thus the standards. Robert

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