4x4" Combination 81EF Color Conversion/Neutral Density 0.9 Water White Glass Filter
The Schneider 81 Series of filters control the bluish coloration that affects daylight film. They can also remove excessive blue from the effects of electronic flash.
Sometimes a Director of Photography may prefer to use an 81EF rather than an 85. When used alone in daylight, this filter under-corrects tungsten balanced film and provides a cool or bluish look. However, if an 81EF is used with tungsten balanced film in tungsten light, it results in a warm look.
It's Autumn. The clouds are low hanging and the sky is "patchy blue". You've gone for a ride upstate to the mountains and the light in open shade is bluish. A photo taken of your girlfriend with daylight balanced film leaning against the side of a barn will yield bluish tones that she might not find particularly flattering.
An 81EF filter (the strongest in the 81 series) will more than replace the yellow missing from the image and place all skin tones on a distinctly "warmish" level. You might even consider using an 81EF at the beach to make a hot scene even "hotter". The 81 series allows you to choose how much warmth you wish to replace in the image.
A Schneider Neutral Density Glass filter has several uses and offers the possibility to achieve otherwise unachievable results. ND filters appear grey and reduce the amount of light reaching the film. They have no effect on color balance.