Re: Dual camera convergence points

[Joel.Alpers@xxxxxxxxxxx]

#
#I often see discussions on the list about dual camera systems for
#stereoscopic photography.  I don't, however,  recall any mention of
#intentionally creating a convergence point.  Is it recommended that the
#cameras be positioned perfectly parallel?

That is correct. If you have a WEB browser, look at the budding
"3D Tutorial" in my WEB page, under "keystone distortion". 

                    http://www.frii.com/~rkymtmem

You'll find a nice writeup by John Bercovitz and Rick Wilson about 
keystone distortion.

When you use a twin-35mm rig, you get a stereo window by eliminating
(during mounting) part of the inner (if I got that right) portion of
each slide.

It's been a _long_ time ago, but we had an in-depth discussion of
how the stereo window is built into 3d cameras. Basically it involves,
not "toeing in" the cameras, because that changes the orientation of
the film plane to the subject, altering perspective and introducing
keystone distortion, but laterally shifting the film plane relative
to the lens. Exaggerated, it looks like this:




                    ___           ___
                   (   )         (   )             (ASCII (tm) lenses)
                    ~~~           ~~~



                _____               _____


Note how the film chips have a larger separation than the lenses. 
This is what sets the stereo window. (BTW if you have a copy of the
PostScript FAQ, it has measurements and stereo window calculations
done on a number of the cameras owned by photo-3d members).

When you do twin-camera 3D, you (should ;)  simply slide the camera 
left and right. Then when you mount, you'll be cropping _more_ of
the area on the right of the left film chip, and on the left of
the right film chip. This will happen automatically when you 
set the window, so you probably won't even notice it, but it happens.

(If you've done any view camera work, this is fairly intuitive, as
the relationship of the _film plane_ (the rear standard) to the subject
determines the _perspective_ and the front standard is used to
control plane of focus.)


Joel Alpers
Rocky Mountain Memories - Equipment and Supplies for the 3D Photographer
rkymtmem@xxxxxxxx
http://www.frii.com/~rkymtmem


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