Diaphragms :
Regulate the opening of the diaphragm of the lens according to :
1. The amount of light on the subject.
2. The speed at which the speed regulator has been set.
3. The H and D speed of the film.
The special Paillard-Bolex table shows the opening of the
diaphragm for most usual subjects. Consult this table often
until you have memorised the information given. The closing
of the diaphragm to the extent of one of the graduations marked
on the scale diminishes by half the amount of light allowed
to pass through the lens. The opening of the diaphragm by
one graduation doubles the amount of light, opening by two
graduations quadruples it, and so on.
Important ! — Opening of diaphragm is obtained by turning
the ring towards a smaller figure (2.8 is a greater opening
than 4). It is closed down by turning it to a higher figure.
Diaphragm opening and speed :
It should be borne in mind that altering the speed alters the
time of exposure. Thus, when filming at 8 frames per second,
the time of exposure is doubled. The normal time of exposure
is 1/30 second at 16 frames per second, which is the normal
speed for shooting. When the film is run at 64 frames per
second, the exposure is reduced to a quarter of the normal
i.e. to 1/120 second. Set the diaphragm of the lens accordingly.
Examples. — If the speed of shooting is doubled by changing
it from 16 to 32 exposures per second, open the diaphragm
by one stop on the scale. If the rate is quadrupled, open by
two stops. On the other hand, if the film runs at 8 frames
per second, close the diaphragm by one stop.
Exposure meter:
The use of a good quality exposure meter is recommended
as a help towards the avoidance of irregularities in exposure.
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