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[MF3D.FORUM:1002] Re: Mounting tips?


  • From: "David Lee" <koganlee@xxxxxxxxxxxxx>
  • Subject: [MF3D.FORUM:1002] Re: Mounting tips?
  • Date: Mon, 31 Jul 2000 22:23:54 -0700



----- Original Message -----
From: David W. Kesner <drdave@xxxxxxxxxx>
To: Medium Format 3D Photography <MF3D.Forum@xxxxxxxxxxxxx>
Sent: Monday, July 31, 2000 8:34 PM
Subject: [MF3D.FORUM:1000] Re: Mounting tips?


> David Lee writes:
>
> > If the window placement really affected the way the deviation was
> > experienced by the viewer, then anytime there was an excessive deviation
one
> > would just have to mount it so that the excess was in fromt of the
window.
> > Discounting the problem of incorrect window placement this obviously
could
> > not be a solution to the problem, because the amount of deviation would
not
> > have changed.
>
> Sorry David, but you are wrong here. Window placement can have
> an effect on deviation. If I had two film chips where the
> deviation between the near points is 2.0mm when the far points
> are homologous and I then mount (place the window way foward)
> so that the far points are 2.0mm apart it would spread the
> near points to 4.0mm. There is still only 2.0mm between the
> near and far points, but there is 4.0mm total deviation
> between homologous points.

It seems to me you are confusing deviation with far point separation. It
doesn't make any difference how far apart the far points are in determining
the deviation. The deviation is simply the difference between the far point
separation and the near point separation (unless you have a different
definition that I am not aware of). So moving the chips apart may change the
far point (and the near point separations) but it is changing them by the
same amount, so the deviation remains the same.


>
> This "technique" is used all the time by projectionists to
> reduce the excess deviation caused by enlargement on the
> screen. They simply move the chips (projector lenses) closer
> together which brings the scene through the window or off the
> screen as some prefer it to be called.

We're talking about apples and oranges here. Projectionists may do this all
the time, but what they are doing is changing the far point separation on
the screen, not the deviation (certainly not the "on film" deviation which
has nothing to do with the window.) You should have said that they do this
to reduce the excess infinity separation. You could take 2 identical chips
(do we agree that they would have no deviation?) and you could separate them
to the point where they would be hard to fuse. Would you then say that they
had too much deviation (of course not, there is no deviation). Or you could
take 2 chips with extreme deviation (say near point at 2 feet and far point
at infinity using a stereo camera) and make it possible to fuse the far
points without making the deviation tolerable.

> So yes, you could correct excess deviation by placing the
> excess in front of the window as long as the total on either
> side did not exceed your personal amount (2.0, 2.3, 2.7, etc.).

Again, this doesn't make any sense to me. The simplest way of measuring
deviation (I can think of) is to place the films on top of each other with
the near points super-imposed. Then measure the separation of the far
points. If I'm not mistaken this is the deviation and it has nothing to do
with any window placement. If you insist on telling me I am mistaken again,
please be sure to include your definition of deviation so I will be able to
understand what you are talking about. It's possible that both of us are
right using our own definitions but that one of us has the wrong definition
of deviation.

For what it's worth, here is Ferwerda's somewhat confusing definition of
deviation,"...the difference between the infinity separation and the
separation of a point in the foreground increases as the point in the
foreground comes closer to the camera. This difference from the infinity
separation is called the stereoscopic deviation, shortly deviation, of the
foreground point."

Doesn't say anything about window placement. Deviation is something that is
determined when the image is exposed and has nothing to do with how you moun
t it. The determinants of deviation are the distances between the camera and
the near and far points, the focal length of the camera lenses, and the
separation between the camera lenses.

David Lee