The 86M with medium thread #29 Dark Red Filter from Tiffen is a filter designed to build contrast in black and white photography and separation between tones. In landscape photography, clear skies are darkened and when combined with a polarizer, a very extreme contrast is added to the image. Many forensic photographers use the #29 Red filter as it absorbs blues colors and enhances red. Therefore, the filter aids in finding evidence. Infrared images are also enhanced giving a stronger effect and look by making foliage look white. The red filter is ideal for copying blue prints, since it enhances the lines and is easier to view the image. It has a filter factor of 16.5, so you must increase your exposure by 4 1/3 of a stop to compensate.
Tiffen filters are not dyed, but are manufactured by using ColorCore technology. This process laminates the filter material between two pieces of optical glass and grinds down and polishes the glass. The glass is then mounted to the metal ring, which after that can be screwed onto the lens for the desired effect.