P3D "Image overlap" problem in macros?

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

With the discussion on stereo macros and different rules for
determining the stereo base, someone should mention the no. 1
problem that makes some macro stereos with 1.2 mm OFD impossible.
This is called "insufficient image overlap".

As the magnification increases the image shifts noticeably during
translation.  For example at 1:1 magnification if you shift by
12 mm then the image has shifted by 12 mm too (assuming that the
axes are not converged).  This situation results in a stereo pair
that has only 36-12 = 24 mm of common area of interest.  Therefore
this pair cannot be mounted in a 7p mount but requires a 5p mount.
And, of course, stretching this to an extreme, if you translate
by 36 mm (possible if near and far points are very close to each
other and you are going for the "optimum" :-) 1.2 mm) then you have
a pair with zero image overlap (= two totally different pictures!)

(BTW, this problem is what forces people to use converging axes or
subject tilt at higher magnifications.  High magnification macros
with parallel shift are nearly impossible!)

Anyone who uses formulas to calculate the stereo base whould be
aware of this problem.  The tables or graphs should light up a
warning light if the image shift is 6 mm, 12 mm, 18 mm, etc.
Every 6 mm the user of slide film is forced to use a different
mount.  At the first 6 mm they are forced to 7p mount, the next
12 mm they are forced to 5p mount, at 18 mm they are forced to
4p mount.  After that, they are wasting their time :-)

George Themelis


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