The Tiffen 138mm #9 (3) Yellow Filter is a basic contrast filter, primarily for use with black and white film. This medium yellow filter slightly darkens skies, helps to cut through haze, and improves overall contrast. It is a common contrast filter for black and white photography, producing an accurate tonal range and compensating for the blue sensitivity of panchromatic film.
There is a 2x filter factor with this filter, requiring an increase of 1 stop for normal exposure. 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 10,000th of an inch, and then mounting to precision metal rings.