Spacing a Ground Glass, Math Check

[Steve Hamley]

Folks,

I'm shimming a ground glass in a 8x10 camera back that had a fresnel installed between the ground glass and the lens. The back measures 0.207" and the T-distance of an 8x10 film holder per Alan Brubaker's site is 0.250" with no film. Film that I've measured is 0.007".

So for the image forming surface to be in the same place as the emulsion, the shim would need to be (0.250 - 0.007) - 0.207 = 0.036". This would make the camera back 0.243" thick, which is consistent with another known good 8x10 I have (0.243" - 0.247").

Right?

If anyone has a known good 8x10 back, I'd be curious in what it measures.

Cheers, Steve

[Jim Jones]

I believe the 1951 ANSI specification for T distance is .260" +/-.016". The slot the film rides in is .012". Film may vary from .004-.010". This means the distance from the face of the holder to the film may be from .272" to .244" if the film is perfectly flat. That is not the correct distance to the front surface of a fresnel mounted between ground glass and lens. That distance depends on the thickness and index of refraction of the fresnel. No wonder f/64 was preferred by some photographers!

[Steve Hamley]

Thanks Jim,

I measure film holder rails at about the same, maybe 0.014.

Here's some data from other cameras:

Wehman - 0.243"

Agfa Studio - 0.245"

Deardorff - 0.190" (!!)

Eastman View #2 - 0.255"

I have not used the Agfa, and the Deardorff only a couple of times. I use the Wehman quite a lot - because it's light and stiff, and it typically returns a very sharp negative or transparency.

Cheers, Steve

[Bill_1856]

Just measure the depth of your film holders, and shim the GG depth the same. (Rocket science not required.)

[Steve Hamley]

Well, that's the theory, and it isn't science, but it is at least arithmetic and when you actually start measuring holders and backs, the numbers don't always seem to make sense, or at least they aren't as consistent as one might expect.

I suspect the dimensions are different because different assumptions were used about film, film holders, acceptable tolerances, etc, but that is a guess.

Cheers, Steve

[Robert A. Zeichner]

Folks,

I'm shimming a ground glass in a 8x10 camera back that had a fresnel installed between the ground glass and the lens. The back measures 0.207" and the T-distance of an 8x10 film holder per Alan Brubaker's site is 0.250" with no film. Film that I've measured is 0.007".

So for the image forming surface to be in the same place as the emulsion, the shim would need to be (0.250 - 0.007) - 0.207 = 0.036". This would make the camera back 0.243" thick, which is consistent with another known good 8x10 I have (0.243" - 0.247").

Right?

If anyone has a known good 8x10 back, I'd be curious in what it measures.

Cheers, Steve

If the image was in focus on the film when focused on the GG USING the Fresnel, then you would simply measure the thickness of the Fresnel and make shims = to 2/3 of that dimension. Make certain that the GG and shims now sit on the same pads that the Fresnel sat on.

[Steve Hamley]

Robert,

That's an approach consistent with what I've also noted. Fresnels are fragile, and I was a bit leery of trying to mike it with calipers or a micrometer. So I decided to measure camera backs, especially ones known to produce excellent results in the field.

The conclusion I've drawn is that camera makers don't seem to be using 0.260" - 0.007" "literally". If so, the film would be at 0.0253", and only the Eastman View #2 is as close to this as I feel comfortable measuring, and the Eastman is a project camera I have not actually used.

However, if you "split the difference" of 0.014" film holder rail space (another 0.007") and subtract that from 0.253" you get 0.246", which is certainly within my comfort zone of accurate measurement, and consistent with the Agfa and Wehman.

Cheers, Steve

[Bob Salomon]

You do realize that film sags in the holder?

[Steve Hamley]

Bob,

Yes. That would explain why the back seems to be putting the GG closer to the lens than the ANSI standard and film thickness would indicate. Play in the film rails and sag would then put the film closer to the lens. Right?

Cheers, Steve

[Steve Hamley]

Just for a reference for the thread, I spoke with Richard Ritter about what he uses while discussing other projects, and he said he use 0.245" and that it worked quite well. He also noted that film thickness and film holder guides vary, and the back will change some with weather.

Cheers, Steve