P3D Re: "built in" windows

[George Themelis <gthemelis@xxxxxxxxx>]

Mike asked:

>> What changes within the camera's engineering achieve this 
>> variety of window distances?
 
And Greg replied:
 
> An outward shift of the film apertures with respect to the
> centers of the taking lenses within the camera.  

Yes, that's correct!  For example, for Realist-format cameras
the spacing of the lenses is 70mm while the spacing of the film
gates is 71.2mm.

> Since the image is inverted and transposed on the film, this 
> corresponds to a forward shift of the window from infinity 
> towards the taking position, exactly like pushing the film 
> chips closer together when mounting.  

Again, that's correct!  Pushing the film chips closer under the
fixed mount during mounting crops the outer edges of the image
and results in a stereo window.

> I was looking for my copy of Waack to find the exact equations

> involved, but he seems to be lost in my library at the moment.


Don't worry... You can look forward for the December issue of
the Stereogram :-) where the subject is discussed are part of
the Tutorial on stereo photography.  The subject is the "Stereo
Window."  

It is interesting that the equation is the same that gives the
amount of deviation: FB/I (F is the focal length or better the
distance between the lens and the image, B is the spacing of the
lenses, I is the distance to set the stereo window).  That's why
for the Realist this shift is 1.2mm which puts the stereo window
at 7ft from the camera. 

For high magnifications is is better to use this equation: M *
B. For example if you take a macro at 1:1 magnification and you
shift the camera by 10mm then you have to crop 10mm from each
image to set a proper stereo window.  It would be impossible to
use a stereo camera for 1:1 magnification macros. :-)  The same
equation explains why you cannot use shift to record stereos at
high magnifications.  At M=1000 if you shift the lens by 1mm you
will have to crop 1m (40 inches) from the resulting image! 
Hence you are must use tilt (either of the subject or the lens).
 

=====
George Themelis (DrT-3d@xxxxxxxxxxxxxxxx)
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