SL3D per John B et al

[P3D <PK6811S@xxxxxxxxxxxxxx>]

John B wrote:

>It is perhaps a conceit on my part, but I believe I am the one 
>most associated on this list with an understanding of how SL3D 
>works which does not agree with Bill's understanding but which 
>does agree with most other people's understanding.  Paul Kline is 
>[...]

Oh crud.  I was going to stay quiet this round, hoping someone
would contribute something new and helpful to the discussion.
There is a lot of good stuff in the archives which won't be
repeated now - hopefully - just search for SL3D.

IMHO:

SL3D works.
SL3D is the simplest possible system for capturing two- or full-
     color anagylphs.
SL3D systems can produce great OR little depth-of-field.
SL3D can divide the z-axis (depth) more finely than dual-lens.

The latter claim is based on the fact that depth-of-field
disappears rapidly with larger apertures, putting objects
out-of-focus which the mind correctly interprets as "at
different depth".  Even flatties can do this, though they
don't supply the color clues as to which direction things
are (near or far).  Dual-lens users sacrifice this effect
to get maximum depth-of-field.  For pictures of your family
it is not that important, but for microscopic use, where 
depth-of-field really disappears, it is helpful (it's not
a problem - it's a feature!)

I think most of the advantage that W Carter describes 
- 'lack of artifacts', 'smoothness' - are related to
large vs. small aperture.  An SL3D camera generally is
used wide open (those color masks really cut the light!)
so the scene is captured from a continuous set of perspectives.
Dual-lens cameras are usually stopped down more and the lenses
are physically separated, so there is a 'perspective gap' between
the two apertures.  One could argue that the dual-lens setup
is more like nature's dual-eye setup, however the whole
capture-process-display nature of photography involves a
number of unnatural processes in attempt to present a realistic
impression to the audience - mimicking the biologic system
is not a requirement, nor is it necessarily optimal.

>[...]
>It is possible to put two apertures in a lens instead of one.  All 
>you have to do is take out the iris assembly and substitute a thin 
>black piece of sheet metal with two holes in it.  In fact, some 

I prefer 5.25" floppy disks :-)  The outside covers are easy to cut
and shape, a little emery cloth takes the shine right off, and
smooths the hole edges very nicely.

>essence of stereo photography.  Of course now your big problem is 
>how to separate or decode the two images; they will want to land 
>practically on top of each other and will indeed land on top of 
>each other for an in-focus object.
 
They do land on top of each other, but because they are captured
from different perspectives they won't be identical - a common
mis-assumption.

>You can place these two apertures arbitrarily close to each other.  
>You can make them touch each other; you can flatten the touching 
>sides of each (make them D-shaped) so as to put them even closer 
>together.  In fact, you can split the original aperture right down 
>the middle and still get effective SL3D.  When you calculate the 
>stereobase, D-shaped apertures act as if they were located at 
>their centroids.

Tried all that, affirmed.  The centroid statement is more
of a claim based on a reasonable 'center of gravity' analysis
than actual depth-perception testing, but can't be far off.

Paul Kline
pk6811s@xxxxxxxxxxxxxx


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