Re: Cardboard cutouts and backdrops...

[P3D Gregory J. Wageman <gjw@xxxxxxxxxx>]

I wrote, regarding the decoupling of vergence and accomodation:

>>Is this really a cue that one must "learn to decouple"?  The fact that
>>a stereo slide is planar while disparity is preserved produces the effect,
>>but the implication of "learning to decouple" these is that one couldn't
>>succesfully view a stereo slide in a viewer until then.

Larry Berlin replied:

>*****  I think you missed the point. For some stereo viewing methods, it is
>not at all necessary to decouple any characteristics. The benefit to stereo
>viewing is that you *can* learn to decouple and it adds significantly more
>to the experience. For freeviewing, a certain amount of decoupling,
>specifically the above mentioned qualities, must be learned. Other qualities
>discussed in this thread have nothing to do with freeviewing, but involve
>decoupling other characteristics commonly tied together in normal sight.

No, Larry, I think *you* missed the point.  John used the phrase "must
learn to decouple".  To me, the word "learn" carries an implication of
practice, trial-and-error, conscious effort, etc.  My point is that
if, in fact, one has to *learn* to decouple vergence and accomodation,
that is, if this didn't happen automatically, then one's initial attempts
to view a stereo slide in a viewer would not be entirely successful (since
focus would presumably change with vergence, the slide would go in and
out of focus as vergence changed).  This is however not the case.

>Yes, decoupling this particular set of factors usually has to be learned.
>It's decidedly different from normal vision.

I agree that viewing a slide in a viewer is different from normal vision.
This was never a point of contention.  However I maintain that decoupling
of vergence and accomodation is automatic when using a viewer.  It is
partially a function of the viewing optics, which put the slide "at
infinity", the "rules" of stereo which limit on-film deviation, and the
brain's own natural tendency to keep an image in focus.  In other words,
it isn't consciously learned, it's built in to the hardware and the wetware.

>Viewing a stereo slide in a viewer requires relatively little decoupling of
>convergence and accomodation. Freeviewing the same slide set would require
>decoupling vergence with accommodation to a much more significant degree. 

I disagree.  Viewing a slide in a viewer requires the same type of
decoupling of vergence and accomodation as freeviewing.  In both cases,
the eyes are focused on a fixed plane, while vergence changes depending
upon the depth of the object of interest in the scene.  What is different,
however, is the plane of focus, being at "infinity" for a slide in a
viewer, while at the real distance when freeviewing.

What makes freeviewing so much more difficult is the fact that the eyes
are trained in normal vision to converge when viewing an object at
freeviewing distances, whereas fusing the pair requires one to diverge
the eyes as if looking at infinity.

The optical system of the viewer provides a somewhat more natural
situation of allowing the eyes to be diverged and focused on infinity
when looking at infinity, but they still must maintain focus on infinity
while converging on closer subjects.  A slide with extreme on-film
deviation will cause problems as one's eyes over-converge to fuse an
object, forcing loss of infinity focus.

The only thing one "learns" in using a viewer is to get used to the
somewhat unnatural feeling of doing so.  Freeviewing, on the other
hand, is entirely a "learned" process because of the complete lack
of optical assistance in the requisite decoupling.

	-Greg W.


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