P3D Re: moving images

["Dr. George A. Themelis" <DrT-3d@xxxxxxxxxxxxxxxxxxxxxxxxxxx>]

I received this reply which I believe belongs to photo-3d:

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"Truthfully, when I noticed this phenomena, there were no adjustments 
being made at all, including focus.  (This sounds crazy but it made 
sense at the time as a quick "preview" of the 100+ slides.  They were 
examined more carefully later.)  The motion I refer to was noticeable 
(I think) only because of the dissolve effect which darkens one image 
down whist brightening the next.  If it were strictly a left/right 
shift Iíd attribute it to the offset between the projection lenses, 
but I noticed some images went "up" on the screen and others appeared 
to go lower.  (It was strictly limited to the position on the image on 
the screen.  It was not a change in the perception of depth.)  The 
throw (distance between projector and screen) was set so that the 7P 
images filled the screen width, causing the 5P & 4P images to have a 
more noticeable border around them, which was clearly asymmetric.  
Since these were all in 41x101 (nominal) mounts, I was wondering:

(a)  if there are known differences in how the different mount and 
mask manufacturers centered the apertures.  I have noticed that the RBT 
4P mounts are not centered (the 21mm dimension is vertically 
asymmetric) but I donít have enough "hands on" time with many other 
brands to have seen similar patterns.  Or can this be from people 
masking the factory apertures (and if so, do that many people really 
bother to?)?"
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My reply:

The 4p (and only 4p) mount in the RBT, Albion and some other
manufacturers is asymmetric as you noted.  This was done to
accommodate Nimslo pictures and should be more properly called
"Nimslo mount", not 4p.  The Nimslo prints the LED light which
confirms exposure into the top and if it is not masked out it
would show up in the picture.  So, if you mount a Nimslo slide
you should have the wider margin at the top.  Those who use
the Nimslo mounts to mask other formats have the freedom to
crop the picture any way they like.

Other than this exception, all other mounts are symmetric top to
bottom and they should not be moving in projection, UNLESS if the
maker has intentionally created an asymmetric cropping by using
silver tape or overlapping different mounts.

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"(b) Since the Bracket Dissolve does not use a traditional changer 
(slides are supported on four small, spring-like tabs and require a 
nimble fingered projectionist), is there enough mechanical error in 
this system to cause this effect?"
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My reply:

I don't think so.  There is some slack in the horizontal direction
and the user of this projector is advised to always push the mounts
in one side (or they will appear to be moving laterally).  Also,
the two sides of the projector must be aligned so that the slides
do not jump when the image is switched from right to left.

Finally, there is some small difference in the position of the
stereo window in space between the RBT mounts and the Albions
or other makers. 

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"I meant no disrespect in referring to you as a purist, and I am 
surprised that you would take offense.  Iím sorry.  It is well known 
that you do not gladly suffer fools.  I only wanted to head off the 
obvious questions and remarks about fiddling with the controls.
M Canter"
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My reply:

OK, I understand :-)  But for future reference please note that I 
am not a purist as this word is commonly understood.  I am a very
practical person and I will gladly ignore things which are not
obvious or disturbing.  As the great McKay said: "if you don't see
it, it is not there!" (see some earlier threads on this :-))

George Themelis


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