The principal use for filters with color film is to balance the color of the light source. One range of filters, from bluish to amber, balances the color temperature of the light. For example, a strong bluish filter from this range can be used with regular daylight film in tungsten light to neutralize the color cast. Another range of filters is available in different strengths of the primary colors, blue, green and red, and yellow, magenta and cyan. These can be used to correct deficiencies in either the film (which can change color with age or because of poor storage conditions) or the light source.
Selective filtration is not easy, as most filters simply change the overall color. A polarizing filter, however, is one of the few ways of making a noticeable change in colors. Light from the sun travels in waves that vibrate in all directions, but when light bounces off certain kinds of material - notably glass and water - the waves are reoriented to vibrate in just one plane. This reflected light creates reflections, but the structure of a polarizing filter, which acts like a grid with extremely narrow parallel slats, passes only those light waves oriented at the same angle, and can be used to eliminate reflections. Blue sky also reflects light, and for the same reason that a polarizing filter works on reflections from glass or water, it can darken the appearance of blue sky at right angles to the sun. It also enhances color saturation by suppressing white-light reflections from particles in the air.
An ultraviolet (UV) filter also improves haze penetration, reduces the overall blue cast associated with atmospheric haze and increases color saturation. The effect is particularly marked when using a long-focus lens on a distant scene. Another occasionally useful filter is a neutral-density (ND) filter, which simply reduces overall exposure without altering color. Occasionally, for instance, there may be too much light for a particular shot, as when a slow shutter speed is needed on a bright day to create deliberately blurred movement. Neutral-density filters are available in a range of strengths.
