USING FILTERS IN BLACK-AND-WHITE PHOTOGRAPHY

Filters may be used in black and white photography for the following major reasons:

To make the tones of gray in the final product conform more closely to the visual effects of colors in the original scene.

To provide scenic contrast, such as darkening the sky so that clouds "stand out." You can use a No. 15 or a No. 25 filter for this purpose.

To lighten or darken a color to make it "disappear" or stand out in sharp contrast. An example of this could be photographing an old document that is written in blue ink and has yellowed with age. Here, a deep yellow filter would darken the blue writing, and at the same time, lighten and possibly remove the yellow stain.

Whenever you use a filter, you must change the exposure. The amount of change depends on the sensitivity of the film to the colors absorbed by the filter as well as the quantity of that color in the type of light used. The effects of the filter in terms of exposure correction are given on the film data sheet as a filter factor.

The filter factor may be applied to the exposure by opening the diagram one f/stop each time the filter factor is doubled. Thus a factor of two requires that the diagram be opened one f/stop larger than is needed for correct exposure without a filter; a factor of four call for two f/stops, a factor of eight for three f/stops, and so on.

An easy formula for determining exposure correction when you are using a filter is to divide the film speed by the filter factor and use the results as a corrected film speed on your exposure meter. As an example, when you use a black-and-white film with an ISO of 400 and a No. 11 filter (filter factor four), divide four into 400 and use the results, 100, as the film speed on the exposure meter. When using this method, make sure you return the meter dial to the correct film speed after using the filter.

FILTER FOR COLOR PHOTOGRAPHY

Problems associated with color materials are quite different from those encountered with black-and-white materials. In color photography, the main problem is achieving correct color balance. The principal factor involved is the color temperature of the light source being used to illuminate the subject. This provides a natural appearance to the final product. Filters for color photography are classified as light balancing, conversion, and color compensating.

Light Balancing Filters

Light balancing filters come in two series (not to be confused with a series that indicated physical size): the series 81 (yellowish filters) are used to lower the color temperature of light source, and the series 82 (bluish filters) are used to raised the color temperature of light from a light source. Both series are used when a tungsten light source is used with color film.

Conversion Filters

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 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 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 be determined by reading filter and film data sheets. If your are in doubt, seek help from your ship or base imaging facility.

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 (such as underwater lighting). These filters can be used to compensate for a known color deficiency of an unexposed color film. They also can be sandwiched (layered) when mounting a color transparency to compensate for an off-color hue.