No matter whether you use black & white or color films, filters play a big part in how your image will appear. Filters come in a variety of colors and special applications, and can be of the round screw-in, drop-in, or my personal favorite, the square Cokin-P series filter types. No matter which you use, you must consider the filter factor (FF) of each when determining the correct exposure for your image.
Film exposures on meter readings (in white light) must be increased to compensate for the loss of light through a particular filter. This increase in exposure is called a filter factor and expressed as a number by which you multiply the exposure. For example, a yellow filter has a factor of 2X in white light, so the exposure needs to be doubled by adding one stop (or opening) to your aperture or by doubling the exposure time.
As I’m primarily a black & white photographer, my filters and filter factors include:
Yellow – FF 2X (lightens areas of yellow, orange and reds, and darkens blues)
Orange – FF 4X (lighten areas of orange and red, and produce darker blues and greens)
Red – FF 8X (lightens reds, producing a truly dark effect to the sky with heavy contrast)
Green – FF 8X (lightens green areas and darkens blues, reds, and oranges)
Blue – 8X (lightens areas of blue while darkening reds, oranges, and yellows)
In addition, other filters such as neutral density have FFs based on their densities. These filter range from 0.1 to 4.0 generally, which a 1/3 stop increase in exposure required at each interval. For example, 0.1 = 1/3 stop increase, 0.3 = 1 stop, 1.0 = 3 1/3 stops, and so on.
So go ahead, have fun and explore the whole new photographic world that opens up when you apply some of these filters to your favorite camera/lens combination.