[Leica] OT: ground glass query

[editor at frontenachouse.com (editor@frontenachouse.com)]

Frank, thank you for the information - your explanation is very much what I 
had anticipated, although considerably more analytical than I came up with 
through my own musings.

The reason for the query is that I acquired a home-made 5 x 7 box camera 
that was thrown in as part of the package with an Exakta collection I 
bought.  The device came with a century-old Goerz lens; for a "diaphragm" it 
has a strip with different-diameter holes; and for a shutter, a squeeze-bulb 
that activates the bulb setting and stays open as long as you squeeze the 
bulb.  Despite the rudimentary , it actually takes surprisingly good 
pictures.  My plan is to replace the whole lens/aperture/shutter mechanism 
with a modern Rodenstock from my Linhof and see what kind of results I get. 
Since the Rodenstock is a different focal length, I'm modifying the 
dimensions of the box - hence my query (for which your answer will be very 
helpful).  If I can make it work half-decently I'll post some pics.

By the way, the 135 Elmarit I got from you a year or so ago is very nice - a 
pleasure to use, with excellent results.

Thanks, D/
----- Original Message ----- 
From: "Frank Filippone" <red735i@earthlink.net>
To: "'Leica Users Group'" <lug@leica-users.org>
Sent: Saturday, December 01, 2007 8:20 AM
Subject: RE: [Leica] OT: ground glass query


> The plane of focus is infinitely thin.  Since the objective is to record
> something onto film, the plane of focus is somewhere in or at the surface 
> of
> the film itself.  Film is something like 0.007 inches thick......including
> the "plastic" backing.  So ideally you place the bumpy part of the GG
> exactly where this plane of film lies.  Your camera SHOULD be calibrated 
> to
> do this.
>
> What does all this mean?  In all practicality, focus is measured to the
> front ( the side that faces the image and is bumpy ) of the GG.  This 
> plane
> is exactly ( +/- a tolerance) from the same plane the film holder places 
> the
> film emulsion.  It is also measured from ( I think) the nodal point of the
> lens.  You have no idea of where this is, so all of this just became
> academic if you are trying to use charts and math to place your lens for
> focus.
>
> Now why do you ask?  Let's get to some practical problem data... and
> solutions.
>
> Are you trying to replace a broken GG?
>
> Frank Filippone
> red735i@earthlink.net
>
> Here's the question: In calculating the distance from lens to the ground
> glass, should the measurement be made from the front of the ground glass? 
> or
> half way between the front and the back? or some calculated plane between
> front and back?
>
> The ground glass itself is thick enough that basing measurements on the
> incorrect plane would seriously screw up the focus.
>
> Thanks to one and all who can help me with this.  Rose
>
>
>
>
> _______________________________________________
> Leica Users Group.
> See http://leica-users.org/mailman/listinfo/lug for more information