The 4 x 4" Combination 85C/Neutral Density 0.3 filter from Tiffen is a very unique filter that offers possibilities that other filters don't.
Neutral Density Filter
A solid Neutral Density Filter from Tiffen has several uses and offers the possibility to achieve otherwise unachievable results. It enables the shooter to adjust exposure without affecting color balance. ND filters appear gray and reduce the amount of light reaching the either the sensor of a digital camera or the film plane of a traditional film camera.
Neutral Density filters are used to create some unusual special effects such as capturing the "blur" of the rippling water of a waterfall or the swirling effect of city traffic. Since they allow the use of slower shutter speeds in bright light, it becomes possible to create these special effects that would normally not be possible to capture without the filter. And since they can also help to control your depth of field, they permit you to shoot at wider apertures. Ultimately, their value is in helping to prevent bright, overly washed out images shot in bright light.
85C Color Conversion Filter
The 85C is in a class by itself since it not only balances tungsten film under daylight but produces a cooler effect than its fellow 85 series filter - the 85B. This yellow filter is used to correct differences in color temperature between the recording medium (film/video) and the light source. Ironically, you can also use this filter to add warmth to your image when shooting under daylight conditions with daylight film.