The Formatt 4 x 4" Dark Yellow 9 Filter is commonly used for a noticeable increase in contrast with black and white films. When your black and white images are flat, throw on a contrast filter.
Contrast in black and white is generally controlled by yellow, orange and red filters with red producing the greatest contrast. Moderate changes can be accomplished with yellow which can, generally, bump up the contrast just enough to make the difference between a "muddy" print and an acceptable image.
Formatt, a company with three generations of professional filter-making experience produces every filter to the highest technical standards. The finest materials, together with state of-the-art production technology have resulted in a standard of optical excellence unmatched anywhere else.