[LargeFormat] fresnel screen orientation

[Richard Knoppow]

----- Original Message ----- 
From: "Vincent Dobson" <manitec@bellsouth.net>
To: <largeformat@f32.net>
Sent: Thursday, November 06, 2003 2:49 PM
Subject: RE: [LargeFormat] fresnel screen orientation


> Thanks Richard, good info.
>
>   I have one question.  If I put the fresnel under the
ground glass of my
> Tachihara, that puts the rear of the GG further from the
film plane by the
> thickness of the fresnel.  Does the fresnel compensate for
this in some way?
> I always worried about that but that is the way Beattie
instructed me to do
> when I got the Fresnel.
>
> Vince Dobson
> Visions In Nature
> www.VisionsInNature.com
>
   No the Fresnel does not compensate. Where fresnel lens is
added to an existing ground glass it must be used on the
outside of the glass in order not to displace the glass.
   Backs like the Graflok are made so that the position of
the ground surface of the ground glass corresponds to the
focal plane of the holders when the Fresnel is installed.
   I suppose one could cut down the fresnel and mount it
inside in some way. That would still disturb the light path
somewhat depending on the index of refraction of the lens
material. For average glass it is about one third the
thickness of the glass, it would be less for plastic. I
don't think it would be worth doing.
   Actually, for view cameras having the fresnel externally
mounted may be an advantage because one can take it off.
When camera movments are used, or for wide angle lenses, the
effect of the fresnel lens may be to make things worse.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@ix.netcom.com