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Richard K.
1-Jun-2011, 14:57
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...:rolleyes: ) 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...

Drew Wiley
1-Jun-2011, 15:33
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).

Oren Grad
1-Jun-2011, 15:49
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... :)

ic-racer
1-Jun-2011, 16:08
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.

Richard K.
1-Jun-2011, 16:51
*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?

Is there anyone that does such work? I.e. critically measuring holders and backs and optimizing them?

Robert A. Zeichner
1-Jun-2011, 16:58
what camera are you using?

Eric Woodbury
1-Jun-2011, 17:10
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.

Ivan J. Eberle
1-Jun-2011, 17:34
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?

Leigh
1-Jun-2011, 18:55
Any decent machinist or LF repairer/restorer should be able to correct it to a tolerance of.001"...
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

Jim Jones
1-Jun-2011, 18:57
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".

Drew Wiley
1-Jun-2011, 19:13
There's an older thread somewhere discussing the cure to this kind of thing, and
how to measure for it. I made a special instrument once for checking the plane,
and found an error simply due to some tiny varnish bubbles which were easily
abraded smooth.

John Jarosz
1-Jun-2011, 19:24
Cameras are usually made such that any discrepancy CAN be fixed by shimming. Are you sure you are measuring correctly? Are you measuring the filmholder to the septum or do you have a sheet of film in there too? (you need the film).

Have you measured all your holders? Are they consistent?

I'd "measure twice, cut once" in this instance.

Is the difference between GG and film plane consistent at all four corners? There are other ways the holder or back could be deformed. It could be saddle shaped, potato chipped, or simply warped. All these non planar conditions are very hard to diagnose and/or correct.

You've opened a can of worms for yourself. Now you probably MUST follow this thru until you've satisfied yourself that you understand the geometry of your back and filmholders.

BTW, I'm not challenging your skills. This is a difficult subject to think thru (Warping, non-flat and non planar conditions). Your GG may not be planar (flat) if the back is not - the glass may be deformed to match the condition of the back.

John

Richard K.
1-Jun-2011, 20:08
Thanks for replies , all. In a few days (more like a week!) I will have time to investigate thoroughly. I will be using a proper depth micrometer (6" base) and yes with film in the holders...

Leigh
1-Jun-2011, 21:27
...I will be using a proper depth micrometer (6" base)
A depth mic is not the proper tool for this measurement IM[-H]O due to inconsistent pressure.

A tenth-reading dial indicator on a similar wide base would be much more repeatable.

How do you plan to evaluate flatness? Single-point depth measurements are meaningless without a flat datum.

- Leigh

Richard K.
1-Jun-2011, 21:55
A depth mic is not the proper tool for this measurement IM[-H]O due to inconsistent pressure.

A tenth-reading dial indicator on a similar wide base would be much more repeatable.

How do you plan to evaluate flatness? Single-point depth measurements are meaningless without a flat datum.

- Leigh

Thanks Leigh! So a depth micrometer won't work? Could you please explain a bit more? What is a tenth reading dial indicator? Do I want something like this:

http://cgi.ebay.com/0-22-X-001-DIAL-INDICATOR-DEPTH-GAGE-SET-HARDENED-/350455572520?pt=LH_DefaultDomain_0&hash=item5198c7a828

I was just going to compare the 4 corners (with film loaded) of my holders to the corresponding corners on the GG frame - they should be identical in theory, right??

Thanks for your help!!

Leigh
1-Jun-2011, 22:13
So a depth micrometer won't work? Could you please explain a bit more? What is a tenth reading dial indicator? Do I want something like this:
It's not that it wouldn't work, it's a question of the proficiency of the user. I could do it, but I'm a journeyman machinist.

The problem is the amount of pressure required to get a reading, and the fact that this pressure is generated by torque of the thimble, which is difficult to gauge unless you have a lot of experience. Differences in pressure will result in different readings.

Another concern with a depth mic is that the measuring face rotates as you turn the thimble, which might damage the ground glass at the points of measurement.

The evilpay item is of the correct type, but with two deficiencies for this application;
1) not sufficiently accurate, calibrated in thousandths, you want one calibrated in ten-thousandths;
2) the base is too narrow.
The advantage of the dial indicator for the task under discussion is that measuring pressure is controlled by a spring inside the indicator, and is quite uniform from one measurement to the next over a small range of distances.

As to flatness, consider this (don't actually do it):
Clamp one end of a filmholder in a vise, then put a pipe wrench on the other end and twist that end 90 degrees, so it's perpendicular to the end in the vise.

In this pretzel configuration you could still come up with identical measurements at the four corners.

A proper flatness evaluation requires establishment of a three-point datum (three points determine a plane, by definition), then measuring one or more fourth points to see if they lie on that plane. The gauging apparatus would initially be set up and calibrated on a surface plate, then used to measure the work.

Precision measurements (in the range of a thousandth of an inch) aren't as simple as they may seem.

- Leigh

Richard K.
1-Jun-2011, 22:26
Thank you very much Leigh. Would either of the two items (down left hand side) on the same eBay page referenced work? Which would you recommend, the digital (at $209.95) or the dial (at $47.70) (they both are better than 0.001" resolution)? Thank you!

Leigh
1-Jun-2011, 22:45
Thank you very much Leigh. Would either of the two items (down left hand side) on the same eBay page referenced work? Which would you recommend, the digital (at $209.95) or the dial (at $47.70) (they both are better than 0.001" resolution)? Thank you!
The general rule of thumb is that your measuring instrument should have an accuracy about ten times better than the measurement you're taking. That dictates an accuracy of .0001" for your .001" measurement.

The $209.95 digital would do the job. It has a resolution of 50 millionths (0.00005"), which is twice as good as you need.

The dial type at $69.95 would also meet the specs, for much less money.

It's really a question of whether you're comfortable reading an analog dial or if you prefer digital readout.

- Leigh

Curt
2-Jun-2011, 01:07
Could the surface plate be a machine top like an industrial planer?
Would gauge blocks and a straight edge work for a go no go initial screening?

John Jarosz
2-Jun-2011, 04:48
Measuring to .001 is fine, but keep in mind what you are dealing with. 8x10 and larger filmholders have a tolerance of +/- .016 on the depth from the front surface to the septum. 5x7 is +/- .010 and 4x5 is +/- .007. Manufacturing tolerances the flatness of wood camera backs will be fairly high as well.(especially as the camera gets larger).

A change of a couple of thousandths isn't going to make much difference. Yes, if you 'stack up' all the tolerances they appear to end up with a huge variation in the location of the film plane. In practice this rarely happens because statistics takes over and one error usually compensates for another.

You also have to know the rate of change of the circle of confusion (for your lens/F-stop) as you move away from the ideal focal plane.

This is just another reason why everyone's been taught to stop down to F16 or so to actually take the photograph.

This whole filmplane issue can become an something that consumes the other aspects of LF photography if you let it. My suggestion is not to allow that to happen.

John

Richard K.
2-Jun-2011, 08:30
The problem is the amount of pressure required to get a reading, and the fact that this pressure is generated by torque of the thimble, which is difficult to gauge unless you have a lot of experience. Differences in pressure will result in different readings.


I can see that not knowing when to stop turning could vary the results substantially, but isn't there a point at which further turning just slips?

How does the depth gauge type avoid this problem? Does it work by pushing the probe in as it is lowered? Would a digital (of either type) be as good as the straight analogue/vernier type? Thanks!!

Oren Grad
2-Jun-2011, 08:49
I can see that not knowing when to stop turning could vary the results substantially, but isn't there a point at which further turning just slips?

But that can be way beyond the position that you're trying to measure. You'd be amazed how much flex there can be in the components of a film holder - especially a vintage one - when you start applying even small amounts of pressure.

Leigh
2-Jun-2011, 22:12
I can see that not knowing when to stop turning could vary the results substantially, but isn't there a point at which further turning just slips?
Many micrometer thimbles have either a friction clutch or a click-stop clutch that limits the torque and improves repeatability.

The problem with these methods is that the resulting pressure might be enough to damage the ground glass, as mentioned previously, or to deform the part being measured, as Oren said.

The dial indicator stem is spring-loaded, producing very light measuring pressure. Since its tip does not rotate there's no risk of abrasion of the work.

Measurement technology, including selection of appropriate devices for a given task, is a science unto itself which has filled many books.
You might want to peruse:
Machinery's Handbook, any edition
Machine Shop Practice by Moltrecht, any edition (two volumes)
Inspection and Gaging by Kennedy, Hoffman, and Bond

- Leigh