Response to the zone system and the use of filters

The filter factor stated on the filter is only a rough recommendation, since all depends on the colour sensitation of the film. Today manufacturers (sadly) no longer "code" additional information about the red sensitivity on the box. In the past there was the distinction between orthopanchromatic, panchromatic and superpanchromatic. The red sensitation is important for the tonal rendition AND the effective speed especially with orange and red filters. "Real" panchromatic films and superpan films had a higher than normal red sensitivity which gave them a higher effective speed under tungsten lighting. Many modern high speed films are panchromatic or superpan. Techpan is superpanchromatic. The higher the red sensitivity, the lower the filter factor and off course vice versa. Orthopan films were promoted as having a tonal rendition closest to the human perception under sunlight, but often they needed higher filter factors, although even in this group one cannot generalize. What I want to say in short: The only way to cope with your problem is to stick to one film and test it with your specific filter and find out the correct filter factor for your system. This is not terribly difficult and when done once accurate for a majority of situations. I never had problems measuring through a filter with a TTL system with yellow, although with red filters I can only recommend to bracket. There are many "invisible" factors that can change the image, including the relative humidity. Also one often does not previsualise the shadow enough.

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