Arca-swiss Discovery GG and Fresnel arrangement

"Tom" <tom@localhost> wrote in message
news:10frle73t5cbs38@corp.supernews.com...
 > The ground surface needs to be exactly at the film plane.
All the other
 > considerations can be adjusted as the designer chooses.
Several cameras are set
 > up just as yours is. Others are set up differently.
However, it needs to be set
 > up the way that particular camera was designed to be set
up (best to check with
 > the manufacturer on that) or focus will be off.
 >
 > metchikabool wrote:
 >
  > > Hello, Group
  > >
  > > I use a newly-purchased arca-swiss discovery that has
the gg mounted
  > > behind the fresnel; in other words, the fresnel is
toward the lens. I
  > > bought this camera slightly used. Perhaps the first
owner set it up
  > > this way. Don't know.
  > >
  > > A fellow photographer who is considerable more advanced
than I am
  > > commented that it seemed like a weird arrangement. He
felt that the
  > > fresnel should be behind the gg, not in front. He didn't
see how I
  > > could get a valid focus.
  > >
  > > Any comments on this arrangement from users with this
setup?
  > >
  > > many thanks
  > >
  > > Jack

An added word. The ideal position is as in the Graflok and
other backs with the fresnel lens on the lens side of the
ground glass with its lenticulated sufface against the
ground surface of the GG. The Graflok is designed for this,
and the Arca may be the same. When adding Fresnel lenses to
cameras designed for plain ground glass the fresnel is used
on the viewer side of the glass with the lenticules facing
the ground glass. The other method reduces internal
reflections and puts the lens close to the light source.
However, the performance is not much different in practice.
The arrangement MUST be correct for the camera as Tom points
out for focus to be accurate.


--
---
Richard Knoppow
Los Angeles, CA, USA
dickburk.DeleteThis@ix.netcom.com<!-- ~MESSAGE_AFTER~ -->
 >> Stay informed about: Arca-swiss Discovery GG and Fresnel arrangement 

"Richard Knoppow" <dickburk.TakeThisOut@ix.netcom.com> wrote in message
news:2m6md3Fjlh0fU1@uni-berlin.de...

 > [...] When adding Fresnel lenses to
 > cameras designed for plain ground glass the fresnel is used
 > on the viewer side of the glass with the lenticules facing
 > the ground glass. [...]

Richard, is it then safe to put the fresnel on the viewing side when in
doubt?<!-- ~MESSAGE_AFTER~ -->
 >> Stay informed about: Arca-swiss Discovery GG and Fresnel arrangement 

"jjs" <nospam.DeleteThis@please.xxx> wrote in message
news:10ft4pnqaq6h1c3@news.supernews.com...
 > "Richard Knoppow" <dickburk.DeleteThis@ix.netcom.com> wrote in
message
 > news:2m6md3Fjlh0fU1@uni-berlin.de...
 >
  > > [...] When adding Fresnel lenses to
  > > cameras designed for plain ground glass the fresnel is
used
  > > on the viewer side of the glass with the lenticules
facing
  > > the ground glass. [...]
 >
 > Richard, is it then safe to put the fresnel on the viewing
side when in
 > doubt?
 >
It depends on the camera design. For instance, Graflok
backs depend on the fresnel for spacing of the ground glass.
The reference surface faces out so that if a ground glass is
put in alone, or is put in closest to the lens it is too
close causing a substantial focusing error. This can not be
corrected by any adjustments because the distance of lens to
ground glass no longer matchs the distance to the film.
On cameras which were NOT designed for a built-in fresnel
it MUST be placed on the viewing side.
One can check the coincidence of the ground glass surface
with the film surface on any camera witht a removable back.
It may even be possible to check by removing the lens board
but is more difficult that way. Measure the distance from a
reference surface (it doesn't matter what as long as it is
stable) using a depth micrometer. then insert a film holder
with a sheet of film in it. Measure again, the two
measurements should be identical. If there is a fresnel lens
in place in front of the ground glass the distance to it
should measure a little too short by approximately the
thickness of the fresnel. If it measures the same distance
as the film the ground glass surface will be too far away
and the camera is not designed for this set up.
In theory the effective thickness of the fresnel will be
its physical thickness devided by the ratio of its index of
refraction to that of air (approximately 1.0). Since most
fresnel lenses are made of rather low index plastic the
physical distance is probably close enough, certainly close
enough to tell you if its in the wrong place.


--
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Richard Knoppow
Los Angeles, CA, USA
dickburk.DeleteThis@ix.netcom.com<!-- ~MESSAGE_AFTER~ -->
 >> Stay informed about: Arca-swiss Discovery GG and Fresnel arrangement 

"Richard Knoppow" <dickburk RemoveThis @ix.netcom.com> wrote in message
news:2m82oiFkac5qU1@uni-berlin.de...

 > [... it's about Fresnel placement...]

Thanks, Richard. My question concerned placement of a fresnel on a camera
that never had one. So, I'll place mine on the film side of the glass, with
the lens expressions against the glass. All I really need in this camera is
a quicker means to frame the image. (Old eyes, you see). I'll remove it to
do critical focusing.<!-- ~MESSAGE_AFTER~ -->
 >> Stay informed about: Arca-swiss Discovery GG and Fresnel arrangement 

"jjs" <nospam.TakeThisOut@please.xxx> wrote in message
news:10ftogkeumj4036@news.supernews.com...
 >
 > "Richard Knoppow" <dickburk.TakeThisOut@ix.netcom.com> wrote in
message
 > news:2m82oiFkac5qU1@uni-berlin.de...
 >
  > > [... it's about Fresnel placement...]
 >
 > Thanks, Richard. My question concerned placement of a
fresnel on a camera
 > that never had one. So, I'll place mine on the film side
of the glass, with
 > the lens expressions against the glass. All I really need
in this camera is
 > a quicker means to frame the image. (Old eyes, you see).
I'll remove it to
 > do critical focusing.
 >
Did you mean the above? On a camera that was not built
for a Fresnel it should go on the outside of the GG. I think
that must be what you meant since that also makes it
convenient to remove it when you choose. Fresnels can make
the illumination less even on a view camera when movements
are used since it tends to direct the light in the wrong
direction. They must also be the right focal length. A
single fresnel will work for both normal and long lenses but
not for wide angle lenses where it will make the corners
even darker.


--
---
Richard Knoppow
Los Angeles, CA, USA
dickburk.TakeThisOut@ix.netcom.com<!-- ~MESSAGE_AFTER~ -->
 >> Stay informed about: Arca-swiss Discovery GG and Fresnel arrangement