[tig] TIG :Re Film steadiness

Peter Swinson peter_swinson
Thu Sep 2 11:32:48 BST 2004


Having read rthe thread on steadiness I thought I would throw in my view of
the variance in achieving steady images from film 

Film cameras have a range of means of film image stability.

Least stable are cameras that have a pull-down claw and rely only on
friction and edge guidance to hold the frame in place.
This is the commonest form of 16mm & S16mm cameras and, I believe, is quite
common in 35mm cameras .
Film manufactures go to extreme lengths to ensure that the "reference edge"
of the film and the perforation position are kept 
as constant as possible.
However the "non-reference edge" may be less accurately maintained.
I once heard that Aaton cameras actually guide on the "non-reference" edge,
how true this is I know not, maybe JPB could
come back on this one.

The next level of stability are cameras that insert a pin or pins into the
perf/perfs to hold the film in place.

As John Pytlak suggests 35mm cameras tend to use a full fitting pin in the
reference perforation, yes there is a specific
perforation in the film relative to the image area.. 
In many camera movements there is another pin on the opposite side of the
film that is a full vertical fit but 
undersized horizontally,such that any slight film path or gate skew or even
film dimension tolerance would not cause the film 
to bow or buckle at the aperture.

It is interesting to note that the pinned perfs are not part of the 4 perf
set at the aperture but are the set of perfs just at the 
top of the subsequent frame. This avoids having pins too close to the
camera aperture. 

S16mm of  course can only use one pin, except I was intrigues to read Chris
Noellert's description of the Northlight 16mm 
registration system usin a sequence of three pins.

Now it may seem obvious that pinned cameras will be ultimately stable,
however the stability is limited to the accuracy of the 
pins and their own stability. Assuming the pins and their fit are 100%
accurate, and this is a fair assumption as they are 
slightly tapered to fit snugly into the perfs, then how about their
stability. 

The pins must enter and be withdrawn for each frame, this implies bearings
in which the pins or their support mechanism reside. 
By their nature bearings must have some clearance or they will seize. 
Therefore the film placement accuracy frame by frame is only as good as the
bearings accuracy.

There are many cameras, mostly 35mm that use this pinning system. The
commonest type used to be the "Mitchell Movement".

For the ultimate in shooting stability there is another movement, the B&H
Clapper Gate. 
This is normally used in rostrum cameras, effects printers and versions are
used in some scanners and telecines.

Again pins as described above are used, with similar dimensions and at the
same positions. 
However these pins are solidly bolted to the gate body, they cannot move. 
This clapper gate mechanism literally lifts the film off these registration
pins and places the film on less accurate transport pins 
that move the film one frame and lower it back onto the fixed accurate
registration pins.

This system is, at least mechanically, the most stable film transport
system. 
It's limitation is speed, yes it can run a t 24/25fps but not at higher
speeds, it is somewhat noisy and a real b*****d to lace. 

Now if there are all these camera transport systems, and as John Pytlak
points out the best stability relies on  "cancellation" 
of movement between frames, then each scanner in an ideal world should have
as many gate systems as there are camera systems.
Not practical. 
The next best solution is the B&H clapper gate or a version of it, at least
it will not introduce additional instability, except 
depending on JPB's reply it may be  the last thing you want where the film
was guide by the non-reference edge !

And if the reference edge has any minute variance with the perforations
again the last thing you want is a fully pinned system.

Life with film transports is a compromise. Fortunately modern transports be
they for camera, special effects printers 
or film scanners cater well for a variety of materials mainly because at
all stages film image steadiness has vastly improved.   

The exception that really gets me wound up is Film Intermediate and Print
printing, I know speed and price are king in the 
release market, but oh how those images even at the IP stage wander all
over the place. 

Please, please, let us soon see the day where all features are scanned off
the original negative and shown digitally at 4K. 
Or, at least can we get a pinned printed IP for the digital scan master,
even if the release IPs are rotary printed.

Congratulations to anyone who read all through this.

Peter Swinson
Peter Swinson Associates Ltd
My views are my own, good or bad

       





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