The 81 series of filters controls the bluish colouration that can affect daylight film they warm scenes shot on cloudy days they can also remove excessive blue from the effects of electronic flash these filters are also good for general scenics when the photographer wishes to remove the bluish cast from open shade or just add warmth to the photograph the 81 series is available in grades from 81 through 81ef with 81ef having the warmest effect lee resin filters are made from lightweight op