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Filters for Infra-Red Night Photography
With the kind permission of Mr. Vernon Walker, head of the R. K. O. Special Effects Department, we report below on results obtained by Mr. Walker in a series of recent tests on the new Agfa Infra-Red, type B film.
The test consisted principally of a number of practical camera exposures, employing various blue-absorbing filters to determine the most suitable filter for night effects, considering at the same time the desired degree of contrast required for background process shots.
Mr. Walker prefers a relatively soft negative for this purpose, correctly assuming that the re-photographing necessary in combining projected background with foreground action is comparable to a regular duplicating process, which customarily requires a master print slightly softer than regular dailies or release prints.
The test mentioned revealed that Wratten filter "G" gave the desired results, ideally satisfactory not only as far as the necessary correction is concerned to produce a realistic night effect, but also considering the specific degree of contrast needed for his purpose.
These results are of particular interest as they corroborate findings from experimental studies which our technical division has already made with the older Infra Red, type "A." Mr. Walker also found that any Wratten filter between G and 29F requires practically the same exposure factor (4 to 6) when used in combination with Infra-Red film for night effects in the daytime. There is, however, a noticeable difference in gamma and color contrast, depending upon which one of these filters is used, and it becomes apparent that the contrast increases in ratio to the increase in red transmission of the filter.
Sensitometric measurements, also, confirm this experience, as will be seen from the accompanying list of gamma readings obtained from sensitometric strips made on Agfa Infra-Red, type B, using different filters:
without filter ... Gamma = .72
with G filter .74
21 ....... .76
23A .78
25 . .79
29F . .80
70 ...... .82
Knowledge of this apparent lattitude in contrast should be of real interest and value to every cameraman, because it permits him to more readily control different light conditions. During the introductory period of this film type, we have recommended the exclusive use of the 29F filter for average results, and have purposely failed to make mention of the variety of additional filters available. This was done for the reason that we were anxious to avoid incumber
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