P3D OVERLAPPING IMAGES OLD AND NEW

["Alan Mahon" <alanmahon@xxxxxxxx>]

Overlapping images are almost inevitable with Realist cameras given their
age and the close tolerance of the original design.  Provision for each
frame is 5 perforations, approx. 23.75mm of which 23mm is the width of the
image.  Therefore at best 0.75mm is the gap between images.  Full frame 35mm
advances 8 perforations or 38mm giving an image width of 36mm and a 2mm gap
between images.  Examining negatives from a range of 35mm cameras will
reveal variations in this gap regardless of whether camera is new, old,
motor driven or manual advance.
I have a modern equivalent of the overlapping problem and would appreciate
any opinions others may have.  After a 2 year wait I acquired an RBT X4 from
the manufacturers earlier this year.  This is a pair of Cosina SLRs spliced
together by RBT.  They make a number of other spliced pairs based on Ricoh
and Yashica bodies.  There appears to be a design fault which causes
overlapping images.  Its specification is full-frame 35mm with a stereo base
of 75mm.  Between the two apertures there is space for one frame.  Having
exposed a pair of images the camera advances one frame (8 perforations or
38mm).  After the second pair is exposed the camera advances three frames
(24 perforations or 114mm) and so on.  The separation of the apertures
should therefore be the equivalent of two frames or 76mm so that images are
evenly placed along the film.  The choice of a base of 75mm therefore seems
incorrect and to make matters worse mine is nearer to 74mm.  I get spaces
between images of 2, 0, 2, 4mm repeating.  The consequence of this is that
processors have declined to mount slide film and they object to printing
from negative film because they say that their equipment needs a consistent
gap to find the edge of successive images.  I know mounting stereo slides is
not ultimately a problem because the widest recommended mount has an
aperture of 33mm but why not set the base to 76mm to be consistent with an
advance of 8 perforations (or 38mm)?  I have emailed RBT and reminded them
but can get no reaction from them so their philosophy is a mystery.