Re: Comparing depth reconstruction among varied viewing methods

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

>the conclusion was that the Gold Button's 35mm FL, while ortho for the
>standard Realist FL of 35mm, does not produce pleasing results when
>viewing normal Realist slides (at least for DrT).  

Let's be careful here Paul.  First, there is a big difference in $$$
between the green and gold button.  The later is a collector's item at a
price approaching $1000, which has nothing to do with lenses, etc.  Second,
let me clarify the "does not produce pleasing results (at least for DrT)". 
It is almost impossible be see the full frame even without glasses.  It is
not a matter of pleasing results. It is more a matter of being able to use
the darn Realist gold button viewer! (I once had one but sold it to a
collector ;))


>>FL (mm)    Camera/Viewer/Projector
>>-------  --------------------------------------------------------------
>>35          Realist and most stereo cameras (RBT S1 too)
>>45-55       Common FL for 35 mm normal lenses
>>44-47       Good viewers (Realist, Revere, Kodaslide etc.)
>>50-55       Star D, $3 plastic, most viewers with plastic lenses
>>35          Realist gold button
>>100         4" lenses in TDC projector
>>125         5" lenses in TDC projector
>>250          Common freeviewing distance (10 inch)

I gave these numbers just to put a range of focal lengths on the table.  I
did not use them to directly compare FLs under different viewing lenses.
But such comparison is possible as follows:

When freeviewing, the effective FL is the distance of the eyes from the
image.  When viewing in projection, the effective FL is the FL of the
projection lenses for an observer at projector level.

>I have read reference in the past to the "ortho seat" in the projection
>room.  How far from the screen is the ortho seat with the 4" lens?  With
>the 5" lens?  

The size of the projection screen is a factor.  Think in terms of ratios. 
The roughly 1" square Realist slide is magnified 40 times when projected
in a 40" inch screen.  The "ortho seat" is 40 times the FL of the camera,
which is 1.4 m or 55 inches, which is roughly the diagonal of the screen
since the FL of the Realist is roughly the diagonal of the Realist frame.
This is pretty close to the screen!  (Another suggested way to find the
ortho seat is to come close enough to the screen so that the screen area
fills the Realist viewfinder.)  Note that the projectionist is viewing
the scene with the same focal length as the FL of the projection lenses
irrespective of the image (screen) size.  Shorter FL lenses allow the
projectionist a more ortho view but even 4" lenses (100 mm) is much longer
than the 35 mm FL of the camera. (has anybody heard of 35 mm projection
lenses?)

>John B also mentioned we can easily observe the stretch effect by moving
>a freeviewed pair closer and farther, but does this mean that the distance
>to the pair is directly comparable to the FL of a viewer's lenses?  

Yes.

>freeviewing at 10 inches result in the same apparent z-axis as viewing
>through a viewer with a 250mm FL?  

Yes, except that freeviewing makes the eyes focus close too which affect
the perception of depth.  Instead of freeving a Realist slide, try to view
an anaglyph at different distances.  The sense that I get is that depth
"increases" when I move far away.  Some people do not readily experience
this which, says to me that the perception of depth might not follow
closely the geometrical reconstruction model. 

It's been a long time since I did any stereo calculations so I hope the
above are basically correct...  John B should correct if this is not the
case.

Thanks! -- George Themelis


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