[Leica] OT: ground glass query

[red735i at earthlink.net (Frank Filippone)]

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