5x7 film plane specification and tolerance

[Wally]

I've read the forum posts regarding 4x5 film plane position and alignment with the ground glass, and there's some suggestion that the ANSI spec (.197 inches +- .007 inches) is explicitly for 4x5.

Does anyone know what the specification is for the film plane in a 5x7 camera? Is it the same?


// Wally

[Wally]

I found these discussions in the archive when i looked further (with better search terms):

http://www.largeformatphotography.in...ead.php?t=3803
http://www.largeformatphotography.in...ead.php?t=1320
http://www.largeformatphotography.in...ead.php?t=1228
http://www.largeformatphotography.in...read.php?t=689

and I found my answer in post 6 of the last thread: 5 x 7"= 0.228" + - 0.010

[Peter K]

Does anyone know what the specification is for the film plane in a 5x7 camera? Is it the same?

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

[Jim Jones]

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

That link gives 0.228" +/- 0.010" as the distance from the face of the holder to the surface of the film. ASA Z38.1.51-1951 gives it as the distance from the face of the holder to the surface of the septum.

[Drew Bedo]

For some reason I have always thought that the standards for sheet filmholders were the same whether 2x3 or 8x10 format . . .From this discussion, I gather that there is a slight difference for each format . . .?

While these are facinating detailes to me, I don't know how much this matters unless someone is making their own holders DIY. Except for one of the wide formats or ULF, why would someone choose to DIY a film holder?

[Peter K]

For some reason I have always thought that the standards for sheet filmholders were the same whether 2x3 or 8x10 format . . .From this discussion, I gather that there is a slight difference for each format . . .?

As smaller the negative as more it must be enlarged to get a certain image size. So not only the circle of confusion has to be smaller also the tolerances of filmholders etc.

This is the reason for ULF, the film can be at any position behind the lens.

[jon.oman]

I've been thinkning of building a 5x7 camera, based on my Shen-Hao.....

Jon

[Wally]

. . .
While these are facinating detailes to me, I don't know how much this matters unless someone is making their own holders DIY. Except for one of the wide formats or ULF, why would someone choose to DIY a film holder?

It's not the case that all manufactured film holders are within spec. So if you find that your shots were sharp on the groundglass but soft in the neg, it's best to do a couple of checks.

First check is to photograph a well-lit ruler at an obliqe angle, focusing on the 6" mark (shows my English units prejudice) very carefully - get your loupe out, adjust very carefully, leave the lens wide open to minimize DOF, let the camera stop vibrating for a few seconds and expose. Develop and look at on light table with your loupe and see if it's the 6 inch mark that's in focus.

If it is, you;ve identified a film holder that you can use as a reference to measure the others against. If not, you may need to measure the accuracy of the position of your ground glass. Refer to the link posted by Peter above to see what the distance is supposed to be - the ground glass should be at the same distance as the film plane minus the thickness of a sheet of film.
If the ground glass position seems to be right, you're on to the next step below. If not, get your camera 'fixed' by someone that knows what they're doing.

Now, for measuring the others, a standard depth guage is not the best tool. You want one of those tools that has a sensitive lever that deflects a dial guage. Measure the known good holder against your others and find out which are keepers and which ones to toss (yes, toss. Don't stick some other poor fool with a bad holder by selling it).

Those links I posted of previous forum discussions describe in great detail how to do the measurements such that the measurement method itself doesn't invalidate the measurement: Some measurement tools will actually deflect the measured surface and give an incorrect reading.

[Peter K]

I've been thinkning of building a 5x7 camera, based on my Shen-Hao.....

Jon, mount a 5x7" back on a conical tube with a baseplate like a RF back for 4x5"-cameras. Than exchange the tube with the 4x5" gg-holder and you can use your camera with both film sizes.

Peter

[Drew Bedo]

Wally: Thanks for the info. Now I am a little inscure about my gear.

When you say that not all holders meet spec, do you mean that quality controme varies from holder to holder within a brand, or do you mean that some brands are better than others? Is there a difference between Lisco or Fidelity? Is there a better (best) holder . . .Linhoff?