The Tiffen 4 x 5.65" CC20B Blue Filter is a color compensating (or color correction) filter for use when making color balance adjustments. It can be used to alter the color balance of a scene by placing in front of a light source or the lens of your camera. It can also be used to apply color corrections when used in the darkroom with an enlarger or to help pre-visualize color adjustments when viewing prints.
Blue is the opposite of yellow and can be used to reduce yellow tones or casts within a scene or print. This blue CC20 is more dense than a CC10 filter and less dense than a CC30 filter, and requires a 0.6 stop compensation for normal exposure results.
Tiffen filters are made using ColorCore technology; a process that involves laminating the filter substrate between 2 pieces of optical glass and grinding flat to a tolerance of 1/10,000th of an inch. This filter measures 4 x 5.65" / 101.6 x 143.5mm.