# Thread: Check Your Ground Glass ~ Film Holders for Registration! Simple, Important and Fast

1. ## Re: Check Your Ground Glass ~ Film Holders for Registration! Simple, Important and F

Jim, I don't know, perhaps it was a 1895 decision to make the bicycle cameras even smaller. I have 3 cameras which seem untouched since sold, one GG is signed in tiny pencil script

I will post my thoughts on this and what I concluded with pictures both of and taken with all 4 cameras with T measurements

In a new thread in a couple weeks

Originally Posted by Jim Noel
GREAT IDEA!! Thanks
On another note, why would anyone insert the ground glass backwards?

2. ## Re: Check Your Ground Glass ~ Film Holders for Registration! Simple, Important and F

Originally Posted by Jim Noel
GREAT IDEA!! Thanks
On another note, why would anyone insert the ground glass backwards?
So they can view the image from the front?

3. ## Re: Check Your Ground Glass ~ Film Holders for Registration! Simple, Important and F

Originally Posted by Tin Can
I'm no genius at that, but is seems negligible for 1.5mm soda green as I measured one from 1902 at 3 days ago

https://refractiveindex.info/?shelf=...oda-lime-clear

The converging beam from the lens to the focal plane converges less strongly as it travels through the glass. That means it forms an image on the back side of the glass, that is slightly farther away than where the image will be formed when the glass is removed. The effective thickness of the glass is equal to (actual thickness / n), where n is the index of refraction. Normal glasses that would be used for a ground glass usually have a refractive index very close to n=1.5.

What this means is that if your ground glass is 2mm thick and the ground side is on the back, the image on the GG will be 2mm behind the front side of the glass, but the image in air will be (2mm / 1.5) = 1.3mm behind where the front side of the glass was. This is enough to matter.

If the ground surface is on the front side, you don't need to worry about this, unless there is a Fresnel on the front side, and then you have to do the same calculation but with the thickness of the Fresnel.

I don't have a perfect illustration of this, but it is the same effect as looking through the surface of a pond at a fish - the light bends at the water/air interface and changes the apparent depth of the fish, see https://www.physicstutorials.org/hom...pth-real-depth

4. ## Re: Check Your Ground Glass ~ Film Holders for Registration! Simple, Important and F

This process can somewhat work for quick checks, but the tougher area is the last 20/1000ths" range, especially if there's a gap below the card...

A cheap (\$20) machinist's import depth gauge is better to tell you where you are at...

Don't forget to put film in the holder for proper height spacing...

Steve K

5. ## Re: Check Your Ground Glass ~ Film Holders for Registration! Simple, Important and F

With your explanation the stack up is in my favor, as I am using 120 year old wood cameras as found, 3 with modern holders, one with matching plate holder which is not ANSI spec in any dimension. but nominal 5X7 as marked by maker on DDS, Lens and camera. A historic variation.

If they were Linhof, Sinar or Horseman I would worry

The GG is 1.5mm so a little less deflection, are you positive your math is correct? The formula I posted was vastly more difficult than simple division.

Good enough for what I am doing which is shooting old cameras as is.

6. ## Re: Check Your Ground Glass ~ Film Holders for Registration! Simple, Important and F

Originally Posted by Tin Can
Good enough for what I am doing which is shooting old cameras as is.
What I originally posted is good enough to let you know whether to check further. These threads always seem to devolve into mathematics formulae about the .003%. Do a quick check and then go make pictures.

7. ## Re: Check Your Ground Glass ~ Film Holders for Registration! Simple, Important and F

Originally Posted by Tin Can
With your explanation the stack up is in my favor, as I am using 120 year old wood cameras as found, 3 with modern holders, one with matching plate holder which is not ANSI spec in any dimension. but nominal 5X7 as marked by maker on DDS, Lens and camera. A historic variation.

If they were Linhof, Sinar or Horseman I would worry

The GG is 1.5mm so a little less deflection, are you positive your math is correct? The formula I posted was vastly more difficult than simple division.

Good enough for what I am doing which is shooting old cameras as is.
The formula on the refractiveindex.info page was for the dispersion, giving the refractive index change as a function of light wavelength. That's something a lens designer needs to worry about, but for ground glass focusing it doesn't matter.

For your 1.5mm thick GG, if the ground surface is on the back, meaning there is 1.5mm of glass between the front surface and the image, that is equivalent to 1.0mm of air. Meaning the film should sit 1.0mm behind where the front, polished surface of the GG sits.

Typically the ground surface is towards the front; Jim is right, that the math and optics are then nitpicking and the depth gauge is an important check. With old cameras, sometimes the GG is reversed from where it should be, because someone put it back together wrong, or the GG is seated wrong or whatever, so it's really a good idea to check.

8. ## Re: Check Your Ground Glass ~ Film Holders for Registration! Simple, Important and F

I check almost every camera as my 1929 SC11 was way off years ago

Thank you all!

Originally Posted by reddesert
The formula on the refractiveindex.info page was for the dispersion, giving the refractive index change as a function of light wavelength. That's something a lens designer needs to worry about, but for ground glass focusing it doesn't matter.

For your 1.5mm thick GG, if the ground surface is on the back, meaning there is 1.5mm of glass between the front surface and the image, that is equivalent to 1.0mm of air. Meaning the film should sit 1.0mm behind where the front, polished surface of the GG sits.

Typically the ground surface is towards the front; Jim is right, that the math and optics are then nitpicking and the depth gauge is an important check. With old cameras, sometimes the GG is reversed from where it should be, because someone put it back together wrong, or the GG is seated wrong or whatever, so it's really a good idea to check.

9. ## Re: Check Your Ground Glass ~ Film Holders for Registration! Simple, Important and F

If reversed, then put it in the right way. If reversed is the 'right way' (I don't understand that) then the glass thickness times 1.5. 1.5 mm glass is about 0.06 inches which 'looks' like 0.09 inches instead. Fresnels don't matter if they are after the image plane, which is the ground glass.

Jim, this is a great little system, but when I tried it last month on my camera, the back of the GG frame didn't seat the way yours does. It wasn't as simple as putting a straight-edge across the back. I went the other way, removed the glass, installed a film holder w/o darkslide, and measured to the plane of the glass from the holder with a caliper. This works well too. I was 0.015 short and added a shim all the way around.

Years ago I checked my 5x7 Deardorff. It was over 0.030" out in one corner and fine in another. I re-machined that back in an endmill to get it flat and true.

10. ## Re: Check Your Ground Glass ~ Film Holders for Registration! Simple, Important and F

Don't forget about the lack of accuracy due to your level resting on raised peeling and blistering paint. It's was a tiny flake of paint undetected underneath the measuring device when the original Hubble telescope mirror was made that cost them billions of dollars to fix! Fortunately, it's a lot cheaper to fix a camera back. To check my ground glass plane, I use a long piece of Starrett ground precision bar stock with a depth micrometer mounted through a hole in the middle. \$20 and \$40 plastic import depth readers are about as accurate as plus or minus the thickness of a smashed sowbug, if not a living one.

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