In checking out my SSG, I've built a back with a Nikon F 52mm lens reversing ring (centered on .220" thick plywood paneling with glassless filter rings) so I can mount my Nikon DSLR and 35mm film camera to test shutter speeds and focusing accuracy. An FG camera's the only one I've got with a split image rangefinder which I think will be very accurate to use for focus accuracy. The lens board has to be about .020" forward of the back limits in order for the lens to focus at both camera split image and the SSG's rangefinder coincidence at infinity.

Which brings up where the film plane is on my SSG. I've seen where the back of the film holder should be 5mm (.197") back of the film holder's front plane. Film's about .203mm (7 to 8 thousandths so says web sites, mine's .008"), so the actual film plane (top of the emulsion ) would be about 4.83mm (.190") back from the front plane of a film holder where it contacts the Graflock back.

I measured my two Riteway and one Graphmatic film holders and all three have their film emlusion plane 4.63mm +/- .0051 (.1822"+/-.0002). I measured 16 places on each one in a 4x4 matrix all over the film to get the overall readings. My focusing back measures 3.50mm (.138") back from the four corner flats to the front of the field lens; don't know how thick the field lens is but probably a bit less than 2mm (.051").

I'm quite pleased at how uniformly flat and parallel all 10 film planes in the holders are to the plane of the film holders where they mate with the Graflock back. But what about them all being about .2mm (.008") closer to the Graflock back plane than on-line specs I've seen.

Comments, explanations and help's welcomed. Something ain't right. It could be me. I've got a old ground glass with split image rangefinder from a Nikon F I'm making a film plane focusing tool that'll be better than the FG body. It's imaging surface will be placed at whatever specs are for the SSG's image plane behind the Graflock back or film holder front plane. It'll be used to test the SSG's rangefinder for focusing accuracy more precise than using a loupe on the focusing back's ground glass.

Bart

Film holder specs:
http://home.earthlink.net/~eahoo/page8/filmhold.html

Kodak TriX 320 sheet film is .007 inch thick. It is listed as "Thick Base".

Graflex mounted the fresnel in front of the ground glass and positioned it so that the ground side of the ground glass is at the optical "film plane". That translates to approximately 1/2 the thickness of the fresnel that the ground surface of the ground glass is further back than it would be if the fresnel was not there.

Remove the back, a major disassembly on the Supers. Insert a film holder and remove the darkslide. Use a straight edge across the back frame insuring that the straight edge is not on a raised position on one side only and measure to the back plate of the film holder in several places. Remove the film holder and repeat the measurements. The ground surface of the ground glass should be the same distance as the film holder back plate. If you use the same film all the time or all the film you use is the same thickness then insert a sheet of film and measure to the surface of the film.

45PSS, thanks for the link to film holder specs. That's a good help for me.

Surprised to see the tolerances at what they are; thought they'd be at least half that much.

45PSS wrote:

Graflex mounted the fresnel in front of the ground glass and positioned it so that the ground side of the ground glass is at the optical "film plane". That translates to approximately 1/2 the thickness of the fresnel that the ground surface of the ground glass is further back than it would be if the fresnel was not there.

Does this mean the "film plane" is half the thickness of the fresnel lens back towards the ground glass; in the middle of the fresnel lens?

Or do you mean half the thickness of both the ground glass and fresnel lenses combinded which is about at the ground glass surface?

I think I'll mount my old SLR ground glass with the split image rangefinder at the same plane as the emulsion on the film is. Measuring how thick the focussing back's fresnel lens is will tell me how far past the .138" the fresnel lens is from the back's flats at the four corners.

Took some shutter test shots with my DSLR on the special back I made. Not too good at 1/30th to 1/125th but decent at 1/250th to 1/750th. Light wasn't not good. And the streak wasn't consistantly bright sometimes. Maybe the shutter leaves are hanging up and not moving the same all the time at the slower speeds. Got to rig an LED on a spinning disk for a better light source.

Bart

The bosses that the ground glass sits on in a Pacemaker Graphic back need to be ground down 1/2 the thickness of the fresnel so that proper film plane position is maintained. I remember this bit cannot find the printed text to reference you to. This is for the " Ektalite Field Screen" used by Graflex.

I would therefore assume that 1/2 the thickness of the fresnel back from the lens side edge of the fresnel is the correct film plane.

45PSS, thanks for the details on the focal plane. I found the following:

http://photo.net/large-format-photography-forum/00LwW3

http://www.largeformatphotography.info/forum/showthread.php?t=65249&page=2 (post 16 lists a site with film holder info)

....that talks about some of this stuff.

I'll remove the parts of the focusing back then mic the fresnel lens. Half its thickness will be added to the dimension from the bosses to the Ektalite Field Lens' front surface. That's what I'll compare to the distance the film is from the front of the film holders; .138".

'Tis interesting to me that the "real" focal plane for the image is half way between the ground glass surface where it touches the ribbed fresnel rings on the Ektalite Field Lens and the front of that Field Lens. It must be how the light rays refract through the Field Lens....or something like that.

I appreciate your help. All this stuff's interesting to me; I do like technical stuff with optics. And I'm learning more about that old Graflex I recently acquired.

Edited to correct error in measurements.......Ooooopppppssss!!!!!!

My dial indicator had the button head set wrong. All those measurements on film holder at .182" to the film emulsion are really .189" +/- .001". Much closer to the spec of .197" depth from film holder flat to bottom of the film holder minus film thickness of .008" which is .189". I guess +/- .001" in film flatness is acceptable.

Bart

http://graflex.org/helpboard/viewtopic.php?t=2628
is the post containing a reply from a very reliable source about the ektalite field screen mounting.