Use with b&w films to create dramatic sky effects, simulate "moonlight" scenes in midday (by slight underexposure). Excellent copying filter for blueprints and low contrast old photographs.
Use with infrared film for extreme contrast in the sky area, also turns foliage white, cuts through fog, haze and mist. Deeper red than the #25.
F/stop incease: 4 1/2 stops use with b&w films for increased contrast or with color film for special effects.