Amber conversion filters help convert cool or blue light to warmer light. The 81A filter is an light amber filter that corrects 5500K light to 5300K. This is helpful on overcast days or shady environments where the light tends to be cool and a little warmth is needed. It is a critical tool for Film photographers because film has a fixed light balance that should be corrected in-camera for the most accurate results.
Cokin filters were invented by professional photographer Jean Coquin. By using square filters like Cokins, which fit into a holder and are attached to a lens via an adapter ring, the photographer has a number of immediate advantages. There is no need to duplicate filters to fit lenses of different sizes, up to three filters can be used at once to create unique effects, and the photographer has control over the positioning of the effect on the photograph.