Re: PHOTO-3D digest 1377

[P3D Neil Harrington <nharrington@xxxxxxxxxxxxx>]

Gregory J. Wageman writes:

>Neil Harrington writes:
>
>>One minor correction:  The lens axes ARE parallel, I believe.  You're
>>correct that they aren't centered on the film apertures, so that the viewing
>>angles of the lenses are slightly toed in, so to speak.  But if the lens
>>axes were not parallel there would be some keystoning.  Granted, the
>>keystoning would be very small with small angles of toe-in, but no purpose
>>would be served by doing this anyway.  As long as the frame apertures are
>>spaced slightly farther apart than the lenses, the camera will have slightly
>>converging angles of view (taking each view as a whole) while the lens axes
>>remain parallel. 
>
>Are you sure that's correct?  I thought the reason for the lenses not
>being centered on the aperture was for window placement, by causing each
>frame to include a slightly offset view of the scene.  I do not believe
>there is any intention to converge the angles of view as you say.

Sorry, Greg, but I just can't see a distinction there.  Yes, the field of
view of each lens is slightly offset, as you say.  The offset is inward for
both lenses, so that the centers of their respective fields of view cross a
few feet in front of the camera.  If that isn't converging the angles of
view, I would sure like to see an example of what converging angles of view
might be like.    :-)

You bet, it establishes the window distance (subject to adjustments through
mounting, of course).

>If the apertures were centered, wouldn't the stereo "window" be at
>infinity, with the entire scene appearing to be in front of it? 

Yep.  Sure seems so to me.

>John Bercovitz writes:
>
>>You guys are saying the same thing, I think.  The axes of the 
>>lenses themselves are parallel or you would indeed get keystoning.  
>>However, lines drawn through the centers of the lenses and through
>>the centers of their corresponding images are not parallel.
>
>Yes I think you're right that we were saying the same thing, but I think
>you expressed it more clearly.
>
>Perhaps it's picking nits, but I think it's a bit confusing to use the
>words "toe-in" or any such thing, because that's neither the intention nor
>the result of the arrangement.

Greg, I can't even SEE that tiny a nit.  

>If I understand it right, the offset lenses simply cause the left image to
>have more information from the right side of the scene, and the right image
>from the left side of the scene.  

Beyond the window, yes.  You don't see that that means their angles of view
must converge?  (Of course I mean angles of view as if seen from the center
of each frame.)



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