Basic principles of projection
Part one : 35 mm Film
projection requires an exposure of 24 frames per second. Each frame is exposed
two times in order to suppress light flickering. For 35mm film this means that
for 1 minute of film a length of 27,36 meter of material is needed.
of film contains almost 53 frames which is sufficient for 2,2 seconds of
In former days projectionists changed over from one projector to
a second every 20 minutes in order to assure a continuous projection without any
breaks or pauses. The projectionist is warned by the first change-over mark to
start the second projector. Sound and image is changed over at the instance the
projectionist notices the second change over mark. Each mark is a small circle,
solid fill most of the time, in the upper right corner of the image on the
screen. Once you are used to detect them, you will always see them. A main
feature of 100 minutes, which is quite normal, takes 2736 meters of 35mm film.
The famous BEN HUR lasts 212 minutes, approximately 6000 meters of film. The
weight of such a film would be 46 kilogram.
Films deteriorate in time.
A fine article on film preservation and storage
can be found in Film Storage
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