GG to film plane discrepancies

[David Honey]

My camera (4x5) is a Graphic View II, for which I've just ordered a Satin Snow GG to replace the old scratched plastic screen.

Likely discrepancies between the GG plane and my film holder planes had been on my mind, and while anticipating the arrival of my first pack of film, I thought I'd check things out..

I began by checking the film plane to front mounting surface of my film holders, and found they are consistently .010" under the ANSI standard (0.197, +/- 0.007"). No matter, I thought, as long as I can shim out the new GG to match my film holders..

Removing the existing 'GG', I found it was made up of two parts -- a plastic Fresnel screen, and a plastic GG. The strange thing was that the Fresnel was in front of the GG!

Now, I think from what I've read that this is incorrect, that the image-gathering surface (i.e. GG) should be the first thing that the image photons encounter. That is, unless my old camera is strange in this regard. Maybe ideas about GG focusing and the position of Fresnel screens were different way back when..?

Anyway, as things stand, the locating plane for the GG in the glass-holder is a full 0.050" (1.27mm) closer than the film surface in my holders. It's a pain in the butt, but I can probably cut strips of gasket material and shim the new GG out by that amount, and all will be fine. I'm guessing 0.050" would make things quite a bit of out-of-focus. (I hate to think how many old cameras out there have no relationship between their GG and their film holders).

Apart from that, am I missing anything?

Oh yeah -- can you actually have the Fresnel in front of the GG? (Moot point in the case of my old camera, as this still doesn't bring the two surfaces to within tolerances!)

And after all this, of course, the film sits a thou or so above the holder plane -- or wherever it wants(!) Just add that to the +/- 0.007" manufacturing tolerances, and it's wonder anything is infocus!

[Alan Davenport]

Apparently, there were (and are, perhaps) some manufacturers who deliberately place(d) the fresnel on the lens side of the groundglass. That's OK as long as they compensate for the focus shift that comes from placing the fresnel in the light path. That might account for your measured discrepancy. The shift in focus would be something like 1/3 the thickness of a planar element (like a filter) but I don't know if that's correct for a fresnel, which is after all a lens element...

I think if it were me, I'd shim the new groundglass to focus correctly without the fresnel, and then add a fresnel behind it if desired. ("Behind" meaning, on the photographer's side of the GG.) A fresnel behind the GG doesn't affect the focus.

[David Honey]

Hi Alan. I don't know whether the setup I described (Fresnel on lens side of GG) is the original one; maybe. Anyway, I've removed the Fresnel and put in the plastic GG temporarily -- shimmed out to match my film holders. A pretty simple fix.
One thing I can say about a LF camera -- it's big -- and nice and easy to work on!

(Now that I have my 'new' 90/8 wide-angle mounted, maybe I see why you might want a Fresnel lens, or at least a better GG!)

[David Honey]

Ha! I naively forgot about the 'flatness' (or lack thereof) of aforementioned plastic 'GG' !

It's got a .006" dish in the middle, convex side towards the lens, made worse by the spring steel clamps that press down on the screen very slightly in towards the center.

With luck, it's curvature might approximate that of the film!

But I'll be holding my breath now waiting for the new GG (real glass) to arrive. Hard to believe this dodgy setup might have been with the poor camera it's whole 50-years of life...

[Dan Fromm]

David, that fresnel was factory issue. It is intended to sit in front of the GG, i.e., between GG and lens. The bosses in the focusing panel on which the gg + fresnel sit are sized so that with the fresnel in place the GG will be in register with the film. Just put the fresnel in position, grooves towards the GG, put the new GG on it. In other words, be calm and leave well enough alone.

[Jim Rhoades]

David, Dan is absolutely right. Graphic made their later cameras with fresnel in place. Try it as is with the new Satin Snow. Camera's with fresnels on the outside were not designed for them is the first place. ie. Wisner and his web site advise. If it ain't broke, don't fix it. Test everything before jumping on ideas from the internet.

[Emmanuel BIGLER]

Oh yeah -- can you actually have the Fresnel in front of the GG?
You can of course but you do not have to.
Arca Swiss ground glasses are mounted like yours with the Fresnel lens in front.
The rationale behind this is to protect the fragile plastic Fresnel lens from scratches when you focus with a loupe. The Ground Glass is often... made of glass but not necessarily 'ground' (could be acid-etched).
So follow the good advice by Dan Fromm, keep the Fresnel in place and everything will be OK.
The Fresnel lens does not care for what kind of GG follows.

[Michael S. Briggs]

As the others have said, cameras can be designed to have a Fresnel in front of the ground glass. With such cameras, you cannot just remove the Fresnel and substitute a plain ground glass. Here are two previous disscussions:

http://largeformatphotography.info/lfforum/topic/499244.html and http://www.largeformatphotography.info/lfforum/topic/503378.html.

Camera owners who add or remove a Fresnel in front of a ground glass are probably the leading cause of major focusing errors. It a good idea to test the focus of any used camera in case some previous owner has made modifications. For most photographers, a photographic test will probably be easiest.

The 0.007 inch manufacturing tolerance for film holders, or even occasional small devations above it, are not causes to wonder how anything is ever in focus. That tolerance is well thought out with respect to typical LF photography. Using a circle of confusion of 0.1 mm for 4x5 photography, the depth of focus at f16 is +/-0.063 inches -- nine times the film holder tolerance. Even if you want to halve the circle of confusion and photograph at f8, you would still have a depth of focus that is twice the tolerance.

What brand and age of holders do you have that are out of tolerance? All of the holders that I have measured have been well within tolerance -- modern Fidelity, Lisco (same factory) and Toyo. So if the 0.010 inch deviation bothers you, I suggest buying newer holders.

[Paul Fitzgerald]

Hi there,

David, the fresnel is standard factory issue BUT the grooves face the lens to work right. They are usually extremely accurate for focus with most holders.

Have fun with it.

[Kevin Crisp]

David: You've received some good advice. Leave the fresnel in front, grooves toward the lens, put the satin snow behind it, ground side forward. Sometimes as they age, the original gg's do get blotchy. The fresnel was a Kodak "ektalite" panel and it really is quite decent. The whole distancing is designed to have the fresnel where it was for accurate focus. If you change it around, you are creating a problem then spending your time trying to fix it. That part (the graflex fresnel and gg assembly) was in production for decades and like so many things Graflex, it was well designed and well built. If you still think something is off, I'd look to the filmholders and not change the camera to increase accuracy. There are endless threads on making sure the distance to the film is what is should be. Having used film holders in three sizes, from modern ones to those from the 1920's, I have yet to have a focus error I can attribute to the filmholder vs. my laziness in not getting out the loupe or my neglect to lock down a camera control. No doubt the filmholders vary considerably from sample to sample, but when you stop down to f:16 to f:45 it doesn't seem to matter. If you number your holders and keep notes you will be able to detect a problem and trace it to a filmholder.

[David Honey]

Gentlemen -- thanks for all your input. I have to confess a stupid error regarding the film holders. I had measured my film holders with a sheet of film loaded when I was measuring for comparison with the GG. For checking tolerances of the holder itself, it should have been empty, of course (duh!) The sheet of film measures roughly .010". So, nothing wrong with the film holders (Fidelity Elites) or the measurements, just my momentary absentmindedness. Sorry for the confusion.

Regarding the Fresnel lens. Unfortunately, this one is pretty scratched up. I thought I would much rather see the image on a nice clean ground glass. Shimming out the new GG to exactly match the position of the film plane looks to be a 'no-brainer'.

A Fresnel screen would seem to complicate things, given it has a theoretical focal length and characteristics of it's own. Not everybody uses one. I've read differing opinions on it's placement, effectiveness, etc etc.

Anyway, the additional Graflex info has persuaded me to at least give the old Fresnel screen another try when the new Satin Snow GG arrives. At least I'll have a flat GG! I'll see where I go from there.

Thanks again everyone.

[David Honey]

PS -- Dan, regarding placement of the Fresnel screen, you said 'grooves toward the GG', others are saying 'grooves toward the lens' ??

[Dan Fromm]

David, Graflex put the grooves facing the GG, not the lens. Other manufacturers may do/have done otherwise.

[Kevin Crisp]

David: I have an untouched factory original, I will check which side the grooves are on and let you know. I thought they faced forward but I may be wrong about that. I think you will find that damage to the fresnel doesn't make a lot of difference if you have a decent gg, and the satin snow is very nice in this set up.

[David Honey]

Thanks Dan, Kevin.

[Kevin Crisp]

The fresnel lens goes in front of the ground glass. The smooth side of the fresnel faces forward; i.e., toward the back of the lens. The ridges on the fresnel face back. The ground glass is installed frosted side forward, up against the ridges of the fresnel. The thickness of the ground glass doesn't much matter, since the thickness of the fresnel places the frosted side of the gg you focus on in the right place.

[David Honey]

Thanks for checking, Kevin.

[David Honey]

Kevin, I reinstalled the GG/Fresnel assembly, as per your instructions. Will try it as is, pending arrival of new glass.

As I mentioned earlier, the plastic GG screen is dished, convex side towards lens. Assembled, with clamping pressure, it's even worse. A straight-edge from corner to corner of the GG with a feeler gauge underneath indicates a .020" depression in the center. Given it's function (theoretical flat plane), it seems crazy to have chosen plastic for this part. (Please, don't let anyone try to tell me that a .020" error in the middle of the film plane is 'nothing to worry about'!)

Looking forward to the arrival of my new, *flat* ground glass...