mounting to infinity

[bercov@xxxxxxxxxx (John Bercovitz)]

Bruce M. Rosenberger writes:
 
> I'm reasonably certain that I understand 'mounting to the window',
> but I've never heard of 'mounting to infinity'  Could someone briefly 
> explain what this means?
 
Well it's a simple enough concept but I doubt I'm a good enough writer to 
tell you in under 100,000 words.  8-)
 
An ideal 3D system would mimic the actual geometry presented to the eyes 
by the real world.  When you look at an object located at infinity, lines 
drawn from your eyes to that object are parallel up to any distance from 
you that we're likely to be concerned with here.  To mimic this, it would 
be good to have the lenses of the 3D viewer spaced the same as your eyes 
and the two film chips spaced so that images of an object at infinity are 
also spaced the same as your eyes.  Now every thing is lined up.  Your 
eyes, the lenses, and the images are all the same distance apart and when 
you look at the images, they appear to be at infinity.
 
First modification:  
As it turns out, your eyes look straight ahead at whatever the lenses are 
looking straight ahead at.  So if the infinitely-distant object's images 
are spaced 63.5 mm apart, and the lenses are spaced 63.5 mm apart, then 
regardless of the spacing of your eyes, your lines of sight to the images 
are parallel.  
 
Second modification:
Your eyes are not very sensitive to how much they are converged unless 
they're converged a lot.  Amount of vergence is not primarily how they 
determine distance.  So if you space viewer lenses at 65 mm apart and 
space the infinitely-distant object's images at 63.5 mm apart, the 
geometry won't override and tell you that the thing is nearer than 
infinity.  This is useful because nothing is made perfectly and that 
includes viewers and stereo slides.  By having the slides at 63.5 and the 
viewers lens at 65, you can allow a generous tolerance to the construction 
of both and still never run the risk of actually making someone's eyes 
diverge, a very uncomfortable state of affairs for the uninitiated.
 
If it's OK to do all this to the system and the viewing isn't wrecked, why 
have standards at all?  Primarily the answer is that it's uncomfortable 
for the audience if one maker's slides are spaced at some given distance 
and then the next maker's slides are spaced at another distance because 
when you go from one slide to the other, the eyes have to find a new 
vergence to infinity.  In addition, if the projector is set up to put 63.5 
slides on the screen properly (the infinitely-distant object's images are 
spaced apart like the eyes) then when a slide with 67 mm spacing is 
dropped in, everyone's eyes have to diverge and a lot of them won't so 
they'll lose fusion.  To some degree, the above statements are also true 
for a viewer.
 
As you notice, I keep referring to the infinitely-distant object's images.  
If you don't have any infinitely-distant objects in the view, you then 
should at least worry about the most distant object in the view.  You 
don't want it to cause divergence either.
 
Comment:
Although your eyes are not very sensitive to how much they are converged, 
they aren't totally insensitive and I for one find it more comfortable if 
the chips are spaced reasonably correctly regardless of whether or not 
there are any infinitely-distant objects in the scene.  It's almost 
undefinable when they're not right.  It just sort of gives one a funny 
feeling and a slight loss of "realness".  Finally, for my medium format 
slides, which are never projected, I have the lenses at 67 to match my 
eyes and infinity at 66.5.  I can get away with this because the slides 
can be made to closer tolerances because I'm doing them myself.  This is 
one comfortable viewer for me to look in.  It feels very natural.  (Of 
course it doesn't hurt that the viewing lenses are a fair match with the 
taking lenses.)
 
John B


------------------------------