Comparing depth reconstruction among varied viewing methods

[P3D <PTWW@xxxxxxx>]

Re: Gabriel thought he remembered an earlier discussion by DrT as to
Green Button Realist viewer having shorter FL than the Red Button
Realist viewer, and DrT's observation that people tend to prefer the
longer FL.  George responds that Red and Green have similar lenses,
and speculates comparison was $3 viewer vs Red Button--DrT's favorite
topic? ;).  I remembered this discussion as comparing Realist viewers
also, and upon searching older digests found that it was actually a
multi-step comparison of $3 to Red Button to Gold Button.  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).  The $3 viewer has
even longer FL lenses than the Red Button, and thus introduces even
more stretch.  The following information provided by DrT in digest 1748
seems pertinent to the current discussion:

>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 do have to wonder about the validity of direct comparison of FL under
different viewing methods.  As most anyone who has seen projected images
knows, depth reconstruction varies tremendously when the person viewing
the projected image changes position along the z-axis in the projection
room.  John B mentioned that "changes are linear so you can easily
compute any combination of changes you like,"  but it certainly would
not make sense to me to say that freeviewing results in a doubling of
the apparent z-axis as compared to projecting with 5" lenses.  (Or did I
totally misinterpret what John B meant by linear changes?)  In projection,
it sounds like either the FL of the lens is irrelevant, or we have to
account for at least two variables when talking about depth reconstruction.

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?  Does the "ortho seat" result in depth reconstruction similar
to what would be observed with an ortho viewer such as the Gold Button?

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?  Does
freeviewing at 10 inches result in the same apparent z-axis as viewing
through a viewer with a 250mm FL?  Has anyone tried this?  And is the
apparent z-axis when freeviewing 5 times the apparent z-axis when viewing
through the $3 viewer?  Stepping away from the pure theory, does anyone
else have real world observations of the apparent z-axis when freeviewing
slides compared to $3 viewer or Red Button viewer?

For me, there was quite obviously less apparent depth when freeviewing
than when viewing with a $3 viewer.  What do other people experience?

Paul Talbot


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