CYBERTRAINING

- ARSCIPRO'S WORKSHOP - © 1986-2009 - Jacques Gurfinkiel

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A- The movie camera

05

Motion picture Camera's basic Rules.

Camera's basic rules are based, as we have seen, on taking 24 (or 25) instantaneous pictures per second on a light sensitive tape, in order to construct a movement illusion.
To that purpose the tape (called film, stock…) has rows of perforations which are intermittently fed by a claw feld system. The feeding is combined with a semidisc shutter rotating in front of the camera gate.
If the shutter is opened, the film will be exposed, that's why it must be perfectly motionless. Soon after that the shutter closes and the claws will move the film to the next step, to a position ready for the coming picture.
That cycle will repeat 24 times a second.

Each film picture is called a photogram.

In professional film tapes are 35mm wide ( that's 35 mm film) ; they have two rows of perforations, which means four perforations for each picture.

There are other film widths : 16 mm film, 9,5 mm, 8 mm, super 8 and so on. The wider the film the better the quality. Professionals consider 35 mm film as being standard, nevertheless there are, actually there used to be, wider films (65/70 mm or even wider) to allow exceptional performances.

Fixity
The guide lines we have been exposing allow taking and projecting pictures on the go, but what about catching something still ? One has to project on the screen a motionless picture (like projecting a single photogram, or a slide). In the case we are considering, it is necessary that 24 times per second the mechanism places exactly in the same position, different images of the same content. Now, as there always is a mechanical loose difference, this becomes impossible. We have then a specific movie failure which is the phenomenon of steadiness (fixity).

It won't be seen until the screening takes place and it consists of the all picture's light movement. The point is that this failure, or an even grater one, occurs on the projector. To find out the failure one needs therefore a special test : the fixity test.

The movement is usully a picture's movement ; now, because " movies move " it won't usually matter. Problems may start when you want to do special effects. For the double or multi-exposure the failure causes cut problems : certain parts of the picture move, or something moves which shouldn't, like a title on background.

Quite a few processes have been created and implemented to overcome the steadiness failure while shooting, the main one being the register pin.
When the shutter opens for exposure, the feeding claw leaves the perforation, while spike-like (register pins) feed the perforations, replacing or maintaining the film during the exposure in exactly the same position

Thanks a lot to Rose Ben for this translation - 3/11/03