The Tiffen 138mm CC20M Magenta 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.
Magenta is the opposite of green and can be used to reduce green tones or casts within a scene or print. This magenta CC20 is more dense than a CC10 filter and less dense than a CC30 filter, and requires a 0.3 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, grinding flat to a tolerance of 1/10,000th of an inch, then mounting to precision metal rings.