[LargeFormat] fresnel screen orientation

[Richard Knoppow]

----- Original Message ----- 
From: "Vincent Dobson" <manitec@bellsouth.net>
To: <largeformat@f32.net>
Sent: Thursday, November 06, 2003 8:44 AM
Subject: [LargeFormat] fresnel screen orientation


> Hi,
>
> I just bought a camera with a horseman fresnel screen.
They had the thin
> fresnel sandwiched under the groundglass on the film side
but with the
> fresnel groves next to the groundglass.  I thought the
fresnel groves should
> be toward the film, but it has been a while.
>
> Which way?
>
> Thanks all.
>
> Vince Dobson
> Visions In Nature
> www.VisionsInNature.com
>
  What you have is the ideal set up where the back has been
designed for it. This is also the orientation of Graflok
backs. The idea is that this arrangement, fresnel surface
against the glass ground surface, has the least internal
reflection and the least spreading of light after the
fresnel. Most camera backs reference the ground surface of
the ground glass so the fresnel must be placed on the back
of the camera in order that it not shift the focus reference
plane. Where the fresnel is on the back the grooves should
face the ground surface of the ground glass to minimise
internal reflections and scattering. So the rule is that the
grooved side should fact the ground surface whatever the
location of the fresnel.
  The idea of the fresnel lens is that it focuses the light
normally scattered by the ground glass surface back toward
the eye.

---
Richard Knoppow
Los Angeles, CA, USA
dickburk@ix.netcom.com