Re: Depth reconstruction in freeviewed images

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

Paul Talbot writes:

>DrT once posed a question to the effect of why do people who know how to
>freeview prefer freeviewing to other viewing methods.  Given this loss of
>depth, such a proposition becomes most mystifying indeed! 

Paul, by freeviewing a Realist slide you are placing it at a distance of
250 mm or longer.  That's 5 times the focal length of your $3 viewer. 
That's a VERY long FL.  My comments on freeviewing were about prints.  The
difference is that prints are larger than slide.   When freeviewing prints
(in magazines) or computer images, one advantage is that you do not notice
the dot/pixel pattern.

I talked about the appeal of longer FL, but within a reasonable range.  For
example, when comparing 44 mm vs. 50-55 mm FL lenses.  Using a 250 mm FL
(essentially done when freeviewing slides) is a very different story.

>If I am correct in deducing from past comments
>that Larry works mostly with freeviewed images, while DrT and myself
>normally view stereo slides in a viewer or projection, this choice of
>viewing method could go a long way toward explaining why some of the
>recent opinions about hyperstereo have been so sharply different.

I think you are correct.

>Has anyone explored this depth reconstruction issue with respect to
>freeviewing vs viewer-viewing of stereo slides?

One major difference is the "effective" FL.  Our resident expert (John B.)
has spoken in these words:

>Mismatch of focal lengths results in changes in the reconstructed depth 
>only.  Mismatch in base results in equal changes in all three dimensions.

I  know this has been discussed in great length in the past, but I think it
is due for a new round of comments.  My specific question is, how does the
mismatch of focal lengths affect reconstructed depth?  In particular, what
effect does the use of significantly longer FLs (generally the case of
freeviewing or in projection while sitting way back) has on reconstructed
depth?  I was under the impression that the z-dimension was stretched. 
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That's why portraits look terrible when viewed with longer FLs (projection,
etc.)  Paul's comments seem to imply that very significant mismatch leads
to flattening.  Is that because we cannot see the deviation any more? 
What's is going on?

George Themelis


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