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

  • From: "Tony Kilbane" <tonykilbane@xxxxxxxxxx>
  • To: <pure-silver@xxxxxxxxxxxxx>
  • Date: Thu, 24 Nov 2005 17:06:35 -0000

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


I liked your simple method of testing whether the ground glass and the film 
plane are within acceptable tolerance. When you have a fresnel screen in front 
of the ground glass, a correction must be made for it's depth. Have you any 
figure for the depth of the average fresnel screen?

Regards,

Tony Kilbane.
  -----Original Message-----
  From: DarkroomMagic <info@xxxxxxxxxxxxxxxx>
  To: PureSilverNew <pure-silver@xxxxxxxxxxxxx>
  Date: 23 November 2005 16:12
  Subject: [pure-silver] Re: [OT] Filmholders, Septums, Total-Disasters


  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 
doesn?t 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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