Contrast control with multigrade and variable contrast black & white photographic papers can be achieved in several ways. One of the most widely used ways is through the use multigrade or variable contrast hand filters.
A magenta filter absorbs green light and transmits blue; A yellow filter absorbs blue light and transmits green. In this way, high and low contrast images can be made.
Ilford produces twelve Multigrade filters that are numbered 00 - 5 in 1/2 steps, with the lowest filter number corresponding to the softest contrast.
Filters are available in sets of filters 3-1/2 x 3-1/2" (8.9 x 8.9cm) and 6 x 6" (15.2 x 15.2cm ). They can be used above or below the lens and can be cut to fit the enlarger filter drawer.
Filter sizes 12 x 12" (30 x 30cm ) are available to special order.
Multigrade filters are very easy to use: no complicated calculations are needed when changing from one filter to another. The exposure time for filters 00 - 31/2 is the same; that for filters 4 - 5 is double.