A filter gets its color because it passes light of its color more readily than other colors. Colors that are distant from it in the spectrum (red, orange, yellow, green, blue, indigo, violet) are attenuated by the filter. This allows black-and-white photographers to use a filter to render parts of a subject relatively lighter or darker, depending on whether their colors are spectrally near to or far from that of the filter.
To make up for the partial blockage of light by filters, manufacturers supply numbers called “filter factors,” by which the exposure time is to be multiplied. If these numbers are 2, 4, or 8, it’s easy to lengthen the shutter speed 1, 2, or 3 stops. However, it’s not useful to multiply a shutter speed by a number that isn’t a power of 2. Therefore, it’s best to remember how many stops or fractions of stops to open the lens (see figure 1).
After I focus my camera, I estimate the largest aperture that will provide enough depth of field. Making part of the picture fuzzy by opening the lens still farther (to accommodate a filter) is not a desirable option. Instead, I change the film-speed setting on the meter. (I also change the film speed to correct for close-ups or when minus development is planned. Making each of these corrections in the meter as the need arises is a convenient way to avoid forgetting to do one of them. Checking the film-speed setting becomes a part of every use of the meter, which isn’t a burden because a careful photographer would check it anyway.)