What's the difference between 11x14 sheet film and glass plate holder focal planes ?

Does anyone know the difference in standard focal plane depths between sheet film holders and glass plate holders for the 11x14 format? I'm trying to adapt an old 11x14 plate holder to do wetplate collodion ambrotypes in an 11x14 film camera. (I don't assume the difference between 11x14 and other formats will be similar.)

I thought I had it figured out but when I ran a couple tests using enlarging paper at the presumed focal plane (loading the paper from behind with a glass "tablet" backing it and pushing it flat to the corners that hold the surface of the plate at the focal plane) the image appeared unsharp, off by perhaps a couple mm from where I (thought I) had focused on a meter stick. Between my trifocals, the shims I made, the old petzval lens and possible focus shift when using a waterhouse stop, I'm not sure where the error lies.

The plate holder is 1" thick while the film holder is 3/4" thick. The depth to septum in the plate holder is ~29/64" while the t distance in the film holder is ~23/64". So, I guess I need to know the standard thickness of an 11x14 sheet of film and the standard thickness of a photographic glass plate of 11x14 size in order to do the calculation and arrive at the difference in focal plane from the camera's ground glass plane.

Can anyone help me out with the material thicknesses or known difference between the two focal planes?

Thanks very much.

Joe

Hmmm. I may have just figured this out a few minutes after posting the question.

If the depth to septum for the film holder is 29/64 or .3593 and the film is .007 thick (IIRC) that gives a focal plane .3523 down from the holder surface.

Since the glass plate holder septum is at 29/64 = .4531 depth, adding a material thickness equal to a sheet of film @ .007 plus another 3/32" = .0937 (isn't that standard plate glass thickness?) brings the depth from the surface to .3524 which matches almost exactly. So adding a sheet of standard film plus standard window glass above the plate holder septum should bring the focal plane and corners of the wetplate "kit" to the correct depth if I'm thinking about this correctly.

Can someone please confirm the thickness of standard film at 0.007" ?


Thanks.

Joe

The dimension that you need to have in agreement is the T dimension of the front surface of the ground glass (front surface of spring back to ground glass front surface) and the T dimension of the film holder (front surface of the film holder to the septum of the film holder) minus the film thickness which I have heard is .007.

Joe: The ASA standard thickness for 11x14 glass plates is .080 t0 .095 inches. Kodak states in their old publication no. F-5, Kodal Professional Black-and-White Films, sheet film is up to .009 inches thick. I've used much thinner litho film and slightly thicker GAF film. These figures agree fairly well with your measurments. I don't trust paper to remain flat in a film or plate holder. Humidity might cause it to bulge away from the septum or glass backing. A few pieces of double-faced sticky tape might eliminate the potential for this problem.

I don't have the ASA specifications handy for 11x14 film or plate holders, but the distance between holder face and septum face for an 8x10 film holder is .260, + or - .016. The slot for the film is .012. The thickness of the film holder is .594. I would assume that the holder face to emulsion distance is the same for both film and plate holders. These tolerances might seem awfully sloppy to a machinist, but at the apertures commonly used in LF photography, they should have little effect on practical photography.

Joe:

Maybe consider making a spacer 3/32's to insert between the plateholder & GG when you focus - then remove & insert the plateholder.

Thanks for the replies. I made some inserts today to seat where the septum originally was and luckily found some acrylic sheet that was around 0.100" in thickness. I just measured the depth to focal plane in the adapted holder and it comes out to just under 23/64" which matches the film holder t-distance minus the film thickness very closely. I'll test it tomorrow and if it works, post some scans.

Jim, one complication is that I'm trying to shoot at f/4 for several reasons with the old Petzval lens. That's why I need to get the adapter fairly close.

Also thanks for the suggestion about a spacer for the camera Lee.


Joe

Welcome Jim:

Good luck & let us know how it works out for you

About $600.00.