The Cokin Z-PRO 031 85C Color Conversion Filter is used when shooting tungsten film in daylight. The 85 series will produce natural colors when shooting with tungsten film outdoors. A #85 filter will decrease the color temperature from 5500K to 3400K and produce slightly cooler results than an 85B.
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.
An 85B filter converts tungsten film to daylight. An 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.
The Z-PRO Series is recommended for focal lengths of 20mm onwards (in the 35mm format). This filter is made of CR39 Resin which is a reputable lightweight and shock-resistant optical material also used for ophthalmic glasses.