Re: Effect of pseudo-stereo

[P3D Dr. George A. Themelis <DrT-3d@xxxxxxxxxxxxxxxxxxxxxxxxxxx>]

>Now that the topic of pseudo-stereo is on the board, here a couple
>questions I have been puzzling over.
>    
>With one pair, the image
>seemed totally flat when the left and right slides were reversed.

I'll let Larry explain this one ;)

>With the other pair, swapping left and right images
>caused an obvious rearrangement of objects along the z-axis: objects
>that had been in the background now appeared to be in the foreground.

Well, that's what you should expect, shouldn't you?

>Second, how does one determine by looking at the two chips of a pair
>which is the left and which is the right, assuming there are obvious
>edge objects toward the foreground?  If I create a stereo pair with
>the shoot and shift method, carefully guarding against any toe-in,
>an object on the left edge will start to slide out of view when I
>move the camera to the right for the right image.  This seems perfectly
>logical.  

Yes.

>However, I have commercially mounted Realist pairs in which
>the right-side view seemingly captures *more* of an object on the left
>edge than does the left-side view.  

That's because the Realist camera has a built-in image shift to put the
stereo window at 7 ft.  This is achieved by shifting the center of the
film apertures with respect to the center of the lenses.  A built-in
window helps in that there is less image waste while positioning the
stereo window during mounting. 

For objects behind the stereo window, the THREE "L" rule applies:  
The Left eye sees Less in the Left edge.  (or, as you said it and as I 
usually apply it, the right eye sees more in the left edge)

>Why do the two situations seem to produce differing results?

When you shift the camera parallel the stereo window is placed at infinity.
If you converge the camera while you are shifting, the stereo window
is placed that the point of convergence.  If an object is at stereo 
window level each eye sees the same in the right and left scene.
If the object is behind the stereo window, the 3 L rule applies
(as in the case of the Realist)  If the object is in front of the stereo 
window, the opposite rule applies (as in the case of parallel axes
SLRs or in "window violations").

You can set the stereo window at will by properly masking the
two images.  I usually set the window so that the closest objects are
at or slightly behind the window.

Two tips:

First, in case you don't know, the Realist marks the right frame with
a little notch at the top.  (Charles Piper has a table showing the
"witness marks" produced by a varity of cameras)  These were put on
purpose and will help you identify the stereo camera that took
a certain stereo pair.

Second, when you shoot single SLR stereos and you have a choice, it
is a good idea to take the right one first and then the left.  This
way you can freeview the chips before cutting.

George Themelis


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