Conversion filters are used in color photography when a significant adjustment of an exposing light is required to convert the color quality of the exposing light to the color temperature for which a film is balanced.

Conversion filters generally come in two series. The 80 series of filters are blue in color and convert tungsten light to color qualities acceptable for use with daylight film. The 85 series of filters are amber in color and convert daylight to color qualities acceptable for use with tungsten film.

The correct filter to use for a given situation with a given film can most accurately be determined by consulting conversion filter tables in the Photo-Lab-Index or reading the filter and film data sheets.

COLOR COMPENSATING FILTERS

Color compensating (CC) filters are used to adjust the overall color balance obtained from color film, particularly slide film. Without the use of color compensating filters, improper color cast can result.

For cameras, CC filters are normally used to color balance the light from sources, such as fluorescent, tungsten, and mercury-vapor lights, and the bounce light reflected from colored surfaces. They are also used to balance lighting effects under unusual circumstances; for example, underwater lighting. These filters can be used to compensate for a known color deficiency of an unexposed color film. They can also be sandwiched (layered) when mounting a color transparency to compensate for an off-color hue.

Table 3-4. Filters and Exposure Increase for Making Color Pictures by

Whenever possible, you should conduct photographic tests in advance, using the type of light you expect to encounter. Consult the Photo-Lab Index for the most accurate filtration to use for your film, filter, and lighting situations. Table 3-4 provides an example of a good starting point for test exposures. When in doubt, you should use a filter that provides for average correction. For daylight film, you should use a 30M filter with a 2/3 f/stop exposure increase. For tungsten film, you should use a 50R filter and a 1 f/stop exposure increase.

CC filters may be used alone or in various combinations. However, when you use them in combination, the maximum number of filters in front of a lens should not exceed three. More than three filters adversely affect image quality. When combining CC filters, you should avoid creating a neutral density effect. Neutral density is caused when all three of the primary colors are present in the combined filters; for example, a cyan (blue and green) filter and a red filter.

CC filters are available in blue, green, red, yellow, magenta, and cyan. Each color is available in a range of densities. The color and density of a CC filter are identified in the filter designation, such as CC50Y. The CC indicates color compensating, the 50 indicates a peak density of 0.50 to blue light, and the Y is the first letter of the filter color-yellow. The peak density of a CC filter refers to the maximum absorption of the color of light that is complementary to the color of the filter. CC filters are available only in gelatin squares.

The color star (fig. 3-4) indicates various color relationships of color compensating filters as follows:

1. Complementary colors are opposite each other: cyan is complementary to red, yellow is complementary to blue, and magenta is complementary to green.

2. Any one color is a combination of the two colors adjacent to it:

Figure 3-4. Color star.

3. The warm colors are at the top of the horizontal axis. The cool colors are at the bottom.

4. Filters of the same color are added and subtracted normally:

30M + 20M= 50M

10B - 05B = 05B

5.When two filters of different colors and equal densities are combined, the color of the combined filters changes, but the peak density remains the same.

10M + 10C = 10B

10R + 10G = 10Y

6. A filter combination having all three primary colors creates neutral density. To correct this neutral density, subtract the lowest density from each color.