Color conversion filters can be very useful when shooting color film, whether print film or slide film. They give you control over the color cast of the resulting image, making it warm, neutral, or cool looking.
The above photograph was shot under interior incandescent lighting without any filtration, and as a result it turned out too warm looking. Read on to understand why as well as how to counter this effect.
Color film has multiple layers which are each sensitive to light of a different primary color. By its very nature, color film is "balanced" to expect a certain distribution or weighting of color across these layers.
This is referred to as white balance with digital cameras. When you set a digital camera for a particular white balance (or let it decide via auto) the camera will re-adjust the color channels after the image has been captured to make whites white and all other colors neutral.
Color films are commonly balanced for daylight. When a light source other than mid-day sunlight is used the balance of incoming colors can be different and the image can look too warm or cool, or have some other color cast to it.
Some color films, especially print films, can be less sensitive to color casts than others. When I am shooting casually with print film I am less concerned about color conversion than if I am shooting landscape work on slide film.
Do you like the warm or cool results from shooting under different lighting? Well if so, by all means don't bother with color conversion filters! When shooting during sunrise or sunset (the golden hour) I never bother to cool the image down, I want the natural warm look.
But let's say that you are shooting in something other than mid-day sunlight and you want to reign in the color cast and achieve a neutral result, here are some of the more common color conversion filters I recommend using with daylight color film:
Shooting in the sun is what daylight films are for. So why use any filter for a shot like this? Skylight filters add the slightest amount of warming to compensate for the minor cooling influence of a bright blue sky, particularly in shady areas. I prefer to use a 1A skylight filter as "protection" on my lenses rather than a straight UV filter as I never mind adding a hair more warmth to my pictures.
Even if sunlight is your primary source of light, when you are working in full shade the color is much cooler. Here I was photographing one of my sons in an indoor swimming pool which was almost fully lit by windows running the length of the ceiling. Counter the cooling effect of shade with an A-series (for "amber") warming filter, in this case I used an A2 filter (similar to 81A).
Incandescent lighting is much warmer looking than daylight. For heavy cooling under incandescent lighting (good old light bulbs) use a B-series (for "blue") cooling filter. Here I used my B12 filter (similar to 80A or 80B) under the light bulbs in our kitchen to render the white tile and stove top neutral.
European designations for color conversion filters go the B-for-blue versus A-for-amber route, while the American designations for the same filters are 81- & 85-series for warming and 82- & 80-series for cooling.
The European naming is easy, the larger the number the stronger the effect (ex. B12 cools more than B8). The American naming is convoluted with its double-numbering and mixed up strengths (81B warms more than 81A, yet 80B cools less than 80A…).
Bottom line: if you want to break out beyond the simple recommendations I make above, arm yourself with a good conversion table (like the one at the end of Ken Rockwell's filter guide) to guide your advanced color conversion filtering technique.