Any physicists (I used to be one but I've have forgotten everything but the Ricci tensor and Spherical Bessel Fucntions which of course are of no use here...) here that can do a quick figure and tell me if a 0.4mm deregistration between GG and the film in its holder would result in *observable diminishment in definition? Thank you!!
*OK, viewing the neg with a 6X loupe...
When I was 16 I thought my father the stupidest man in the world; when I reached 21, I was astounded by how much he had learned in just 5 years!
-appropriated from Mark Twain
Depends on how much your film sags (ordinary filmholder don't hold film completely flat,
the bigger the film is, the more it sags, and films differ in stiffness anyway), and in how
much your film will be enlarged (what is not noticeable in a contact print might be a real problem with significant enlargement). It also depends on the focal length of the taking lens (longer lenses are less affected by minor focus errors).
t = 2Nc(1+m), with t being depth of focus, N the f-number, c the circle of confusion, and m the magnification. For small m, this approaches t = 2Nc.
Pick your CoC, and off you go...
1/f = 1/p + 1/q
The 'simple lens' formula will tell you your focus shift at say 6 meters*.
The diffraction/defocus formula (thanks Oren) will tell you that even though there is a calculated focus shift, it can be ignored if you stop down enough (assuming a Large Format camera, figuring 1 to 2 mm focal spread at f16).
If it were a small format camera, 0.4mm would not be tolerable.
If it were me, I'd call it too much, even for large format.
*When I test a camera I use this focus shift observed on the film to work backward with the simple lens formula to see how much I need to shim the ground glass.
Thanks all; unfortunately I would need to, um, what's the opposite of shim?...(I need the GG 0.4mm closer) I suppose I could shim the opposite sides of the GG frame where the holder abouts against them?Originally Posted by ic-racer
Is there anyone that does such work? I.e. critically measuring holders and backs and optimizing them?
When I was 16 I thought my father the stupidest man in the world; when I reached 21, I was astounded by how much he had learned in just 5 years!
-appropriated from Mark Twain
what camera are you using?
That's about 0.016 inches out. That is quite a lot. Seems like I recall +/- 0.007" tolerance for a 5x7 film holder, but could be wrong. That goes way back.
Example. 150 mm lens focused at 10 meters is 152.3 mm back focal distance. If you instead had 151.9 mm, you'd be focused 11.99 meters or 152.7 mm would be 8.48 m. You need to fix that.
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It's also potentially cumulative; your other tolerance errors in the imaging chain might add to the error. Any decent machinist or LF repairer/restorer should be able to correct it to a tolerance of.001" so why not fix the one big easily correctible error?
I agree (I r a machinist), with a possible exception.
That tolerance should be attainable on a decent metal camera.
It may not be possible on a wood camera because the back may not be flat.
- Leigh
It might be best to make both the GG frame and the film holders conform to standard diminsions. The distance between the face of standard film holders and the septum is .197" +/-.007" for film holders up to 4x5; .228" +/- .010" for 5x7; and .260" +/- .016" for 8x18. The minimum width of the slot the film rides in is .012".
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