P3D That damned Irishman...

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

That damned Irishman, Alan Mahon, did it again!  He made me think.  So I
took a ruler and pieces of exposed and unexposed film, a paper and pencil,
and did measurements and kept notes.  What I have concluded is that he is
absolutely correct.

Film perforations are spaced 4.75mm apart.  Full-frame cameras advance 8
perforations which is equal to 38mm.  This gives 36mm for the width of each
frame, plus 2mm gap between each frame.  The RBT so-called "75mm" cameras
advance the film 8 and 24 perforations consecutively.

The only way the RBT spacing would work is if the film gates are spaced
76mm apart.

Alan's X4, as well as mine, and also anyone else's I know of (not just X4
but X3 and X2), have this peculiar "problem" where the spacing of frames is
uneven.  Some frames almost touch, others have a regular spacing and others
a wide spacing.  I have noticed a periodicity in the spacing of mine,
roughly 2, 0, 2 and 4mm and the pattern repeats itself.

Alan correctly identified the cause of this periodicity and gave a sound
explanation of the  uneven spacing.  If the film gates are not 76mm apart
but, say 75mm, then instead of each frame being separated by 2mm you get
this sequence: 2, 0, 2, and 4mm.  So one mm difference in spacing of the
film gates throws you into this pattern.

This creates an interesting question: Is RBT aware of this?  

I would imagine that any person or group of people that designed the RBT
cameras MUST be aware of this simple fact.  So, why is this happening?  Why
is the camera designated 75mm spacing of lenses and not 76mm?  Is this 1mm
offset done on purpose?  If yes, what is the purpose?  

It was suggested that the spacing of the lenses might be different than the
spacing of the film gates with the intention to built a stereo window into
the camera.  (For example, the Realist has a 71.25mm spacing of film gates
and a 70mm spacing of lenses.)  I do not have my RBT X4 here to measure the
spacing of the film gates but I would think that RBT makes no effort to put
a stereo window because this would require that the film gates are shifted
with respect to the lenses and I would assume that when you start from two
already made cameras then it would be very hard or impossible to alter the
spacing of the film gates with respect to the lenses.

The other possibility is that RBT really aims for 76mm even though they
quote a 75mm lens spacing but production tolerances results in small
variations that cause this uneven spacing.  That needs to be confirmed.
The fact that all RBT cameras I have heard of, do have a consistently
uneven spacing makes me doubt that this is the case.

The final issue is this:  Does the uneven spacing make any difference?  Is
this a problem for any one?  

Even if the spacing is uneven between some frames, the complete sequence is
consistent as film advance is based on sprockets and it follows the 8 and
24 perforation sequence.  If the lab's equipment are cutting and printing
based on perforations, they should not have any problem.  Alan has reported
problems.  Others do not report problems.

Bottom line:  Alan appears to be on solid grounds when he suggests that the
uneven spacing of film frames in the "75mm" RBT cameras might be the result
of some kind of "miscalculation" from RBT's side.  It would be interesting
to get RBT's perspective into this issue.

I must say I agree with Greg's feelings that it is a miracle that the
cameras can be joined together and work so well as a system.  But it would
be nice if we were assured that the uneven spacing is not a result of
miscalculations but it cannot be avoided for some reason.

George Themelis