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

  • From: Michael Briggs <MichaelBriggs@xxxxxxxxxxxxx>
  • To: pure-silver@xxxxxxxxxxxxx
  • Date: Thu, 24 Nov 2005 15:34:31 -0600 (CST)

On 24-Nov-2005 DarkroomMagic wrote:

...

> 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.

A agree that placing a Fresnel lens in front of the ground glass is undesirable
because of the focus shift, but some manufacturers ship cameras this way.  They
probably make this choice to have the ground glass protect the Fresnel lens. 
I think owners altering their screens, such as incorrectly altering Fresnel
lens setups, is probably a more common cause of mis-focusing than bad holders.

The above description of the focus shift from placing a Fresnel lens in front
of a ground glass is correct mechnically but omits an optical effect. 

The optical effect is that a Fresnel in front of the ground glass shifts the
location of the focus plane.  When the screen is removed to insert the holder,
the focus plane returns to the original location.  So in the configuration 
of a Fresnel in front of the ground glass, you do not want the textured surface
of the ground glass aligned with the film plane, as stated above.

This optical effect occurs when any plane-parallel plate is inserted into a
converging optical beam.  Assuming that the index of refraction of the plate
(Fresnel lens) is 1.5, the shift in focus is 1/3 the thickness of the plate
farther from the lens.

For the original question from Tony about how to make a physical measurement to
verify focus: I think he will have to be able to accurately measure the
thickness of his Fresnel in order to be able to calculate the correction
between the film and ground glass positions.  The simple almost-no-tools method
won't work.  The ground glass should be 1/3 the thickness of the Fresnel behind
the correct film plane.  So the front surface of the Fresnel should be 2/3 of
its thickness in front of the film plane.  So the depth to the front of the
Fresnel should be 0.190 inch - 2/3 the thickness of the Fresnel.  (0.190 is the
film holder depth less the thickness of film.)

It might be easier to make a photographic test of focus.

--Michael
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