1 The Director of Photography – an overview

Colour temperature 123
Figure 14.3 Mired shift
values of Wratten colourcompensating
or CC filters
Positive – towards magenta Negative – towards green
Filter Value Filter
factor
5M – �2
10 M – �4
20 M – �8
30 M – �13
40 M – �18
1
⁄ 3
1
⁄ 3
1
⁄ 3
2
⁄ 3
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⁄ 3
Colour-compensating filters have only a limited use in cinematography,
as they are primarily available for fine-tuning colours in transparency
or reversal work. They do come into their own, however,
when a cinematographer has to work with available light from fluorescent
tubes, which often have an excess of green light. This discrepancy
can be corrected by putting the appropriate magenta filter,
magenta being the complementary colour to green, over the tubes or
in front of the lens if no other source of light is to be added.
Filters have another important value, their filter factor. This is also
shown in the tables (Figures 14.2 and 14.3). The filter factor is always
expressed as the part or whole of a stop on the lens and is expressed as
a fraction of a real number or a real number plus a fraction.This is the
amount that the lens must be opened up to allow for the amount of
light absorbed by the filter. Therefore, a filter factor of one-third will
need the aperture to be opened by one-third of a stop. Similarly, a filter
factor of two-thirds will need the lens aperture to be opened by twothirds
of a stop.
Here it is important to understand the difference in approach when
adding together Mired shift values and filter factors. If you put two filters
in front of the lens then you will combine their Mired shift values,
taking regard of their mathematical sign. For example, combining two
filters having shift values of �35 and �10 will result in a combined
Mired shift of �25. But if their filter factors, when expressed as f-stops,
were one-third and one-third, then the combined filter factor will be
two-thirds and the lens must be opened up by two-thirds of a stop.
Filter factors are always positive so are always added together.
In practice, it is usual to consider changes of 100°K or more as important,
as this is the smallest change the eye is likely to notice unless it is
possible, within the scene, to compare the sources – in which case differences
down to as little as 20°K may be noticeable. This may occur,
say, when a row of windows have one lamp each outside them, representing
sunlight. In this case, very accurate measurement and filtration
of the lamps will be required to get them all within a 20°K range.
The colour temperature meter
Filter Value Filter
factor
5G – �2
10 G – �4
20 G – �7
30 G – �10
40 G – �13
Colour temperature meters come in two basic forms, reading either two
or three colours.The two-colour meter reads only blue and red. It is not
sophisticated enough for a working cinematographer but is useful for
setting filters on lamps, as this usually only concerns blue and red.
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⁄ 3
1
⁄ 3
1
⁄ 3
2
⁄ 3
2
⁄ 3