The Tiffen Series 9 Combination Color Conversion 85/Neutral Density (ND) 0.9 Filter will produce natural colors when shooting with tungsten film outdoors. Using tungsten film in daylight will produce a bluish cast in the photograph.
What do you do when you must take some photos outdoors and all you have left is tungsten film? Using tungsten film in daylight will produce a bluish cast in the photograph. The 85 series of filters will "bring back", i.e. color balance the tones in your images. These yellow filters are used to correct differences in color temperature between the recording medium (film/video) and the light source.
It should be noted that an 85 filter produces slightly cooler results than an 85B (with tungsten film in daylight) whereas an 85B converts tungsten film to daylight. The 85C produces the coolest results - more than either the 85 or 85B. However, it is interesting to remember that an 85C is also useful as a creative warming filter with daylight film in daylight.
Tiffen Neutral Density filters have several uses and offer the possibility to achieve otherwise unachievable results. ND filters appear gray and reduce the amount of light reaching the film. They have no effect on color balance.
Determining which neutral density filter yields ideal results for any given lighting situation takes knowledge, experience and a collection of such filters. Speaking generally, the 2-stop value (ND 0.6) effectively compensates average situations.
Tiffen's ColorCore glass is produced through a process that entails permanently laminating the filter material in between two pieces of optical glass that are ground flat to tolerances of a ten-thousandth of an inch, then mounting them in precision aluminum rings.