The Formatt 62mm 81C Color Conversion Filter controls the bluish coloration that affects daylight film. They 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 81C filter will replace even more yellow than an 81B and place her skin tones on a more "warmish" level.
An 81C can also be used with electronic flash to reduce the amount of "blue" generated by the flash. 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.