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

  • From: DarkroomMagic <info@xxxxxxxxxxxxxxxx>
  • To: PureSilverNew <pure-silver@xxxxxxxxxxxxx>
  • Date: Thu, 24 Nov 2005 19:17:24 +0100

Tony

One variation in ground glass design is the addition of a Fresnel lens. Its
purpose is to provide even illumination over the entire ground glass, making
focusing, especially in image corners, significantly easier. The Fresnel
lens is a flat piece of plastic, with one side built up from a series of
thin concentric rings, which function like a lens. The rings are usually
barely perceptible to the naked eye, but become obvious when viewed through
a focus loupe.

A Fresnel lens will equalize image brightness when placed either in front of
or behind the ground glass, but there are some pros and cons with either
setup. When a Fresnel lens is added to an existing camera back, it is far
simpler to place it behind the ground glass. The ground glass retains its
position, and the alignment with existing film holders is maintained.
However, in addition to image formation on the textured surface of the
ground glass, it is possible to focus an image on the ridges of the
concentric rings of the Fresnel lens. The image formation on two separate
surfaces can make accurate focusing difficult.

Alternatively, the Fresnel lens can be added in front of the ground glass.
This has the advantage of image formation only taking place on one surface,
since the ridges are in contact with the textured surface of the ground
glass. However, the disadvantage is that the ground glass, and the
associated focus plane, is out of its original position. Consequently, the
focus plane is no longer aligned with the film plane, and the camera back
must be machined or adjusted to allow for the Fresnel lens thickness. In
either setup, make sure that the textured surface of the ground glass faces
the lens and is aligned with the film plane, and that the ridges of the
Fresnel lens are facing the ground glass.

I advise against placing the Fresnel lens in front of the ground glass. If
placed behind, no focus correction is required.





Regards



Ralph W. Lambrecht

http://www.darkroomagic.com









On 2005-11-24 18:06, "Tony Kilbane" <tonykilbane@xxxxxxxxxx> wrote:

> Ralph,
>  
>  
> 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
> 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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