1 The Director of Photography – an overview

Film stock 73
Figure 6.13 Kodak’s positive
perforation
One problem in the early days when nitrate base was still being
used was the propensity of the base to stretch when wet in the processing
chemicals and then shrink back, on drying, to a dimension
smaller than it started out. This resulted in a problem when trying to
contact print a shrunk negative onto an as yet unshrunk positive piece
of film – they were not the same length and the perforations were now
on different spacings.
The solution to this problem was to perforate the negative and positive
stock with the separation of the perforations, or their pitch, initially
different, so that after processing and shrinking the negative had
a resultant perforation pitch the same as the unprocessed positive.
While the Bell & Howell perforation was perfect for a negative
stock that requires maximum accuracy and only has to run through
the camera gate once and a printing machine a few times, it was not
ideal for print stock, which has to run through a projector many, many
times. The problem was that, as can be seen in Figure 6.12, the join
between the curved section of its shape and the straight line section
results in a sharp corner, and this corner was prone to tearing after
being run through a projector many times.
A different perforation was then devised, as shown in Figure 6.13,
where the shape is more square and all the corners are rounded. This
gives the positive perforation much more strength, though it is harder
to manufacture the punches for perforating positive perforations to
the same tolerances as the negative perforation. The positive perforation
is known as the Kodak perforation.
Things changed with the advent of triacetate film base, as triacetate
does not shrink after processing anything like as much as the old nitrate
stock. The pitch of the perforations had, therefore, to be changed, so
that with the new base the processed negative and the unprocessed
positive had, again, the same dimensions when coming into contact in
the printing machine.
You may hear reference to negatives having either long pitch, the
old standard, or short pitch, the newer standard. You need not worry
about this when ordering stock, as unless you make a very specific
request it will always come as the modern, short pitch.
The old, long pitch is only used nowadays for very specialized applications,
such as when several film strips are run through the gate of the
camera at the same time – as in some forms of process photography.
For 35 mm, the consistency and accuracy of perforating is vital to
the steadiness of the image. The majority of both camera and projector
mechanisms in use today locate the image using only the perforations.
In the camera, this is usually achieved by inserting register pins
into two or four perforations after the claw has pulled down the film;
they are then left in place for the duration of the exposure, being withdrawn
after the shutter has closed ready for the next pull-down. In a
projector mechanism, the usual arrangement is to have the film pulled
through the gate by a rotating sprocket, which is locked absolutely
rigid for the period the shutter is open.This is usually achieved using a
device known as a Maltese cross. The benefit of using a Maltese cross
mechanism in the projector is that as the film is wrapped around at
least half of the sprocket, many of the perforations are used to pull the
film through the gate and this mechanism is therefore very kind to the
film, so that it may be shown many times without damage.The Maltese
cross has not been used as a camera mechanism for perhaps 90 years,