If your enlarger has a filter drawer, you can simply drop filter squares into the drawer to change the contrast of ILFORD MULTIGRADE paper. If your enlarger lacks a filter drawer, you will need to use mounted filters, by sliding them into an adaptor under the lens. If you plan to make prints with divided filtration, using a below-lens kit reduces the risk of moving the enlarger head between exposures.
Filter transmission values are based on the assumption that a tungsten light source will be used to expose the paper. The optical quality of the filters is high, so they may be used anywhere in the enlarger light path. If you use them below the lens, you must handle them with particular care to avoid finger marks and other blemishes. You can clean the filters with a soft cloth or camel-hair brush.
MULTIGRADE filters are very easy to use, because they are density-balanced: no complicated calculations are needed when changing from one filter to another. Filters 00-3 1/2 require the same exposure; filters 4, 4½ and 5 require twice as much exposure. For example, if a print made with filter 2 requires an overall exposure of 10 seconds at f5.6, a print of similar overall density made from the same negative will need 20 seconds at f5.6 when using filter 4½.
Instead of MULTIGRADE filters, you could use a MULTIGRADE 500 head on your enlarger (see under “Multigrade Heads”) or a colour head (suggested settings for some common models are given under “Colour Enlargers”). In addition, some enlargers (e.g. some Leitz and some Durst models) can be adapted for use with MULTIGRADE by adding a variable contrast module similar to a colour head; and some manufacturers (e.g. Kaiser and LPL) make special MULTIGRADE heads for their enlargers.
However, there are very good reasons why you should still buy and use MULTIGRADE filters. The yellow and magenta filtration controls of most enlargers lack the extreme values that are needed to produce really low and high contrast; and cheaper enlargers have non-linear filtration controls that make consistent fine adjustment of contrast difficult at each end of the scale. Most important of all, though, is the fact that the filtration in your colour head is not density-balanced. Each time you change filtration, you will need to carry out a calculation to determine the new exposure time. MULTIGRADE filters are not expensive, and using them will save you much time and trouble.