GG to film plane discrepancies

[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...