Re: "Perfect" slide mounting?

[P3D Dr. George A. Themelis <fj834@xxxxxxxxxxxxxxxxxxxxx>]

Paul Talbot writes:

>Gary Nored recently mentioned always using an alignment guide to properly
>set far objects, and ignoring window consequences others might find
>objectionable; Lincoln Kamm countered that he favors freedom of stereo
>window choice. 

Let's clarify the terminology here:  Gary favors "mounting for constant
infinity" while Lincoln favors "mounting for stereo window".  The first
method sets the infinity points at constant separation and lets the stereo
window fall wherever it may, while the second method sets the stereo window
at will (usually at the level of the closest object) and lets the infinity
separation vary.

>How does this exchange relate (if at all) to past
>debates about adjustment-free stereo projection?  

It has little or nothing to do with it.  Whether a projectionist chooses to
adjust the projector's vertical and horizontal alignment for every
individual slide or not, has (or should have) nothing to do with the
mounting method preferred by the slide maker.

>(Some have proposed
>that a projector be set once, using a properly mounted guide slide,
>and damned be those slides that do not register 100% correctly without
>adjusting the projector.)  

Because I favor the idea of not adjusting the projector for each individual
slide, I do not like the way this is phrased.  First of all, a slide does
not have to register 100% correctly to be enjoyed.  Only those slides with
gross misalignments present a problem.  The question then is:  Who's fault
is it that certain slides have serious misalignments and should the
projectionist adjust the projector to eliminate those misalignments?  My
answer is that the projectionist should not adjust the projector to
accommodate a mismounted slide because the next properly mounted, slide will
be projected misaligned (and get punished for no good reason) and the audience
will get sick (literally) and tired of the constant adjustment of the
projector.   I have H. C. McKay behind me on this :-)

>In your response, assume that there are no
>vertical or rotational errors in the mounting of the slides.  

If we exclude vertical errors then we are only left with horizontal errors.
But such animal does not really exist, IMO.  During projection we have two
partially overlapping images.   Objects that show zero deviation (complete
overlapping) appear at screen level, objects that show positive deviation
(right frame object in the right of left frame object) appear behind the
screen while objects that show negative deviation appear in front of the
screen.

The usual practice is to adjust the projector's horizontal spacing so that
the mask's edges overlap on the screen.  This brings the stereo window (set
by the mask) exactly at screen level.  Objects that are behind the stereo
window will be projected behind the screen and objects in front of the
stereo window will be projected in front of the screen.  This is the most
natural setup.  For a person mounting for the stereo window, this window is
at screen level and the desired window effect is maximized.  For a person
mounting for constant infinity separation, infinity points have a constant
separation in the screen, as desired.  Everybody is happy.  Why should the
projectionist change this by adjusting the horizontal spacing?  

Some projectionists will adjust the horizontal spacing to eliminate
ghosting.  If a bright/contrasty object is close to infinity (typical case:
sunrise/sunset) then severe ghosting will occur because the two (R/L)
projected images have maximum separation.  If this object is placed at the
level of the screen then the two images overlap and ghosting is eliminated.
The projectionist might try to adjust the horizontal spacing to bring
ghosting-prone objects at screen level (thus throwing everything else in
front of the screen.)  I don't think this is a good practice.  The maker of
the slide should be aware of possible ghosting problems and either not show
the slide or mount it with the ghosting-prone object sitting at window
level.  Of course, this last solution is not possible if one mounts for
constant infinity separation but, as I said, by adjusting the projector to
eliminate ghosting this constant separation is, literally, thrown out of
the window.

Paul, if the above are not very clear now, I am sure they will be clear
once you get your own projector and do your own experimentation.

-- George Themelis


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