The B+W 77mm 81A Slim Filter is a color conversion filter that helps to slightly decrease color temperatures and provide a warming effect. The 81A filter adds more warmth to the image for use with tungsten films in natural lighting. It provides a decrease of 200K, helping to render the scene more similarly to incandescent illumination.
There is a 1.5x filter factor with this filter, requiring an increase of 0.6 stop for normal exposure. B+W filters are constructed from high quality Schott glass for increased optical clarity and color fidelity. They feature a brass filter ring for durability as well as jamming prevention. The slim filters are specifically designed for use with wide angle lenses and are only 0.1" / 3mm thick. They do not have front threads for stacking additional filters.