Basic principles of projection

Part one : 35 mm Film

[film] A smooth 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.
One meter of film contains almost 53 frames which is sufficient for 2,2 seconds of projection.
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 White Paper.
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