I recently vented some "ink" on the misguided photographers who feel filters degrade image quality, but only touched on the many useful and necessary applications of filters. Though I mentioned the long history of use in black and white work, it seems that the near collapse of b & w in the '70s caused a break in that tradition. Many current b & w shooters are purists who feel that the image should be totally unaltered.
That's highly unrealistic of course, and shows the influence of color photography. Yes it's true, a good color photo and good monochrome are wholly different critters and many subjects don't work well both ways. The color photographer lets the various planes and masses separate themselves by color and actively seeks vivid or complementary color schemes. A red rose looks great in a bed of green leaves. Taken on b & w film however, the gray values of both may be almost the same and the effect is very flat, -- the rose is nearly lost. Since clear separation of the subject from its background is one of the hallmarks of a successful photo color thinking doesn't work very well in monochrome.
Hooray, galloping over the horizon, here come filters to the rescue. The deep colored, sharp cutting b & w filters (and by the way, nowhere here am I referring to the B+W brand of filters) can darken and lighten specific colors and restore tonal separation. The general rule is that they lighten their own color and darken complementary ones, but it's easier to just look through them, at the subject, and see the effect. In the above case a red filter could brighten the flower and darken the leaves restoring a dynamic contrast to what would have been a dull image.
Some specific effects; a red filter darkens blue and green but lightens red, orange and yellow; Green darkens red and blue but, of course lightens green; Blue lightens itself and darkens yellow, green and red. These are the primaries, RGB. Remember that yellow is only a subtractive primary in that it subtracts blue only and transmits green, yellow and red. Blue is used the least since the effect is like rolling back b & w film’s progress to the pre1880s when colorblind films left blue skies white and vegetation dark and muddy. The opposite effect is achieved by using any one of the yellow, orange or red filters that b & w workers consider basic to outdoor photography. They give progressively greater darkening of skies and emphasis of clouds -- in fact yellow is considered necessary to just achieve normal and accurate tonality.
Keep in mind that "lightening" colors or tones is only, a relative term. No filter can add light, they only subtract -- some colors more than others. Therefore extra exposure is always needed and the specific “filter factor” is usually packed with the filters. Lacking printed data, a good rule to follow is, add one stop for light ones like yellow, two for medium, like green, orange and light red, and three or more stops for heavy reds and blues.
The special effects filters like crostars, zooms, multiprisms, soft focus and close-up lenses ("diopter lenses") work equally well on color and monochrome but this is one group that can degrade the image to varying degrees. Of course "degrade" may be the actual intent of the filter -- as with soft focus-- but multiprisms also cause annoying color fringes and close-up lenses have slight aberrations, so use them with care (stopping down the close-up lenses usually fixes any loss of sharpness.)
Filters for color have a basically different purpose. They subtract wavelengths only partially, to produce different mixes of the colors -- rather than chopping some out (only the UV filters are sharp cutting). For instance the salmon colored 85 and 85a warm up daylight just enough to make it look white to a tungsten film and the bluish 80, 80a,80b make tungsten light look white to daylight film. The 81 series (a,b,c,d) is prized by
otographers who want to add just a hint of extra warm color, and less known is the 82 series (and I’ve never seen more than the "a”)
which is used to add slight degrees of cooling.
Ultimate color control can be achieved by using the CC (color compensating) filters which come in all six primaries (RGB-CMY) and seven densities from .025 to .50. Specific lighting problems or duplicating films might need any one or several of these but rarely will anyone need the whole set of 42 gels. One specific CC deserves mention and it's the CC.30M (magenta). This one is the best guess for eliminating the excess green in popular cool white fluorescent lights, so it's useful enough to be one of the few CC gels that the Tiffen Co. cements into a glass filter (but at an outrageous price).
A nifty and creative use of filters is called “flash back”. Lets say you're taking a photo of a fellow tourist in some medieval ruins on a gray, overcast day. The straight shot would be an overall cold gray or with a warming filter like an 81a, an overall warm gray. So put a blue #80 filter over the lens to create a bluish, mysterious aura for the background (don't forget to read the exposure through the filter) and put a cheap (even from a “sample book”) #85 gel over your fill in flash. Since the 80 and 85 have directly opposite effects the flash will now flash the foreground back to normal color balance. A variation on that would be to double the 85 on the flash, or use an 85 plus 85c for a halfway effect, to get a warm glow on the foreground which will sharply separate it from the mysterious blue background. Various filter combinations can be used for many color effects but remember that sharp cutting b & w filters cannot be flashed back to a white balance.
Flash back is sometimes a necessary control in big interior shots. For instance people in the foreground in a large, fluorescent lit room may need flash to stop motion. First a CC.30M is put over the lens to clean up the fluorescent light. Since that would make the foreground flash look pinkish it is covered with a CC.30G (green) gel and it "flashes back" to neutral color. What happens if there are windows in the background? Many square yards of CC.30G gel may be needed to cover them -- greatly raising the cost of the job -- or the photographer could wait until night.