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December, 1950 ward : producing 16mm. colour films 193
of colour correction is required. It will be noticed that the phrase " if a variation is detected which is considered undesirable " has been used. This is not intended in the slightest degree to be any reflection on the makers of the stock. Such variations as do from time to time occur are usually slight and can often be ignored, but it may happen that for precision work some correction is desired, for example, where it is desired to secure the very best rendition of a particular object or material. Sometimes the stock manufacturers recommend use of a filter for minor correction of colour balance, but if any filter is found to improve results in any particular batch as a result of one's own tests, it is a useful service to inform the manufacturers so as to make this information available to others.
The filters usually employed for such correction are the Kodak colour compensating series, CC23-24-25 (Yellow), CC33-34-35 (Magenta) and CC43-44-45 (Cyan), and, generally speaking, these will provide all the correction necessary or desirable. Here I would like to project another test reel, in which the effect of various filters was tried in a particular case. The details are as follows : —
Stock. — Regular Kodachrome.
Lighting. — 700 foot-candles, white flame arcs, no filter on lamps. Colour temperature 5,100° K.
Filters and exposures. — As in Table II.
TABLE II Filters and Exposure Values on Second Test
Test number
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15
Filter on lens
Exposure at 24 f. p. s.
—
//2.2
—
7/2.8
—
//3.5
CC14 .
//2.2
CC14 .
7/2.8
CC14
7/3.5
CC15
7/2.2
CC15 .
7/2.8
CC15
7/3.5
81 .
7/2.2 .
81
... . //2.8
81
7/3.5
CC25
7/2.2
CC25 .
7/2.8
CC25 .
7/3.5
The film was then projected.
As a result of these tests it was decided to use a CC25 filter on the lens of the camera in the particular production in question.
Method of Use of Filters.
A further point may be made here, that great care must be exercised before deciding that a correction filter is necessary. The need should only be judged from a properly exposed test ; under-exposure will naturally tend to blueness, whilst over-exposure tends to redness.
Filters used for such a purpose are, of course, always used on the lens of the camera. Where, however, it is desired to introduce correction of the colour of the light sources, whether filters are used on the camera or on the lights themselves depends largely on the individual circumstances. If a scene is illuminated wholly by one type of source, for example, white flame arcs, of constant characteristics, the use of a filter on the lens gives satisfactory results.