The Formatt 3 x 3" 81D Color Conversion Filter controls the bluish coloration that affects daylight film. It can also remove excessive blue from the effects of electronic flash.
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 81D filter will more than replace the yellow missing from the image and place all skin tones on a "warmish" level. The 81 series (81A, B, C, D and EF) allows you to choose how much warmth you wish to replace in the image.
This filter can be used by the pro and amateur alike, weekend photo warrior or pro cinematographer and must be considered an essential addition to any photo arsenal.