[pure-silver] Re: [OT] Filmholders, Septums, Total-Disasters

  • From: DarkroomMagic <info@xxxxxxxxxxxxxxxx>
  • To: PureSilverNew <pure-silver@xxxxxxxxxxxxx>
  • Date: Wed, 23 Nov 2005 17:00:06 +0100

Yes!

I designed a gage to check film holders (picture on request) and checked
over a hundred holders. Many of them are outside of ISO tolerance, new or
old, it doesn¹t matter. Some brands are better than others. Richard wrote a
while back on this list which brands were the worst.

With view cameras, the image is composed and focused on the ground glass.
One surface of the ground glass is textured to provide a means for focusing
the image. It is important that this textured surface faces the lens,
because it is the image forming side. To take an exposure, the ground glass
is replaced by the film holder. At this point, the film must be in the same
plane as the ground glass was during focusing, so the negative is perfectly
sharp. Camera backs and film holders must be machined to tight tolerances to
ensure this condition.

A well-focused image and full utilization of the intended depth of field are
achieved if these tolerances are close to zero. Small deviations can be
tolerated, because the depth of focus for view cameras is relatively large
(1 mm or 0.040 inch for a 4x5 inch film format at f/5.6), but even small
tolerances will shift the focus and depth of field. It is, therefore,
important to keep the ground glass in perfect alignment with the film plane.
However, some of our cameras and film holders deviated enough from these
standards to warrant a simple check.

In his May/June 1999 Photo Techniques magazine article, Jack East Jr.
proposed a simple but effective alternate method to check whether the ground
glass and the film plane are within acceptable tolerance.

Place a piece of film into a holder and insert it into the camera back.
Remove the back from the camera, and lay it flat on a table. Rest the edge
of a rigid ruler across the camera back. Hold a toothpick or cocktail stick
vertically against the ruler, lower it until it touches the film and clamp
or tape it to the ruler, thereby identifying the film plane location. After
doing this with all film holders, leave the toothpick positioned for an
average holder.

Now remove any film holder from the camera back, and compare the average
film plane with the ground glass location. If the toothpick just touches the
ground glass, then no adjustments are required. Knowing that a sheet of
regular writing paper is about 0.1 mm (0.004 inch) thick provides a tool to
quantify any offsets. If the toothpick touches before the ruler, then you
could shim the ground glass with paper, but if there is an unacceptably
large gap between toothpick and ground glass, then professional machining of
the camera back is required.

With the toothpick still positioned to identify the average film plane
location, measure all film holders for variation. According to the standard,
a tolerance of ±0.007 inch, or two layers of paper, is acceptable for the
4x5 format. Discard or avoid film holders outside this tolerance.





Regards



Ralph W. Lambrecht

http://www.darkroomagic.com








On 2005-11-23 16:04, "Michael Healy" <emjayhealy@xxxxxxxxxxxxx> wrote:

> Is there legitimacy to this guy's claim?

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