Filters are usually identified by numbers. This system of designating filters is used to identify Kodak Wratten filters. It uses designators, such as No. 6, No. 8, and No. 11. Some filters have a descriptive name rather than a number; for example, polarizing, skylight, and neutral density. Color compensating and color print filters have yet another designation system.

FILTERS FOR BLACK-AND-WHITE PHOTOGRAPHY

Filters used with black-and-white film are classified as contrast, correction, and special purpose. All contrast and correction filters have a noticeable color. It is important to note that a filter must be used with an emulsion sensitive to the specific color of light it transmits. Colored filters should normally be used with black-and-white film only.

Special-purpose filters for black-and-white film may be colorless, contain a hint of color, be noticeably colored, or almost visually opaque. Some special-purpose filters can be used with both black-and-white and color film. Special-purpose filters are covered later in this chapter.

Contrast Filters

Contrast filters are available in all colors and are designed to exaggerate, reduce, or eliminate specific colors of light. As their name indicates, these filters are used to increase or decrease contrast in a negative that provides differences between tones in the print.

To illustrate this, compare a red apple and a yellow banana in a black-and-white print. With a red filter over the camera lens, the apple appears lighter on the print than the yellow banana. Both objects in this example reflect the same intensity of light.

When you look through a red filter, the filter definitely appears red. This color is the effect it produces in your eye and the reason it is called a red filter. The red filter is transmitting most of the red part of the spectrum, some yellow, and some magenta. The color it is not transmitting is cyan. If you think of this red filter as an anticyan (blue and green) filter, you will better understand the way it works.

When a red filter is used, most of the reflected red light from the red apple is transmitted through the filter and recorded as a dense area on the film. Only a portion of the yellow light is transmitted, so it is recorded as a less dense area on the film. Only some of the yellow light is transmitted because the reflected light from the banana consists of red and green light. Although the red portion of the yellow light is readily transmitted through the red filter, the green portion is absorbed to some degree. Thus less light from the yellow banana reaches the film emulsion.

When the negative is printed, the two print images have separation in contrast because of the differences in negative densities. The print image of the apple is lighter than the print image of the banana because the negative image of the red apple is more dense than the negative image of the yellow banana.

When using a specific color of contrast filter to provide separation between black-and-white images of colored objects, you should also take into account what effect the filter has on the images of other colored objects in the scene. For example, when there are blue and green objects in the scene, the red filter absorbs some or all of the reflected blue and green light. The red filter renders the negative images of these objects as low-density areas. Thus the print images have darker tones or densities.

Contrast filters can also be used to filter out an image or filter out the image of a transparent stain on an original document by copying it. This filtering-out process takes place by blending or matching the density of the image to be filtered out with the image density of the surrounding area For example, to eliminate the image of a yellow line on a white background, use a yellow filter. The yellow filter should be as deep (same color density) or deeper in color than the color of the line. The yellow filter reduces the intensity of the light reflected from the white background by absorbing blue light. The intensity of the light reflected from the yellow line is not greatly affected since the yellow filter readily transmits the yellow light. The reduction of the intensity of the light reflected from the white background and the intensity of the light reflected from the yellow line produces equal densities on the negative and thereby does not render an image of the yellow line. Conversely, when the yellow line is on a black background, a blue filter does not allow yellow light to be transmitted. Therefore, light from the yellow object is not allowed to affect the film emulsion. Thus the line appears as a thin area that matches the black background and is thereby eliminated

Stains on a drawing or a picture can be filtered out whenever the stain is transparent and reasonably pure in color. The filter should be approximately the same color as the stain. The stain may still show in the negative but, in the case of line material, proper paper contrast and printing exposure get rid of the rest of the stain image.

Remember, the color of filter required to eliminate the image of an object or stain is determined by the color of the object or stain and the darkness or lightness of the surrounding scene area. Also, always use a filter that is as deep or deeper in color than the color of the object or stain to be eliminated. Refer to table 3-1 for clarification on ways to use contrast filters.

Table 3-1. Parallel Filter Bars

Use the parallel filter bars to choose contrast filters for black-and-white photography. Adjacent filters lighten colors next to them. Opposite filters darken colors in the print; for example, a yellowish green No. 11 filter lightens subjects that are yellowish green or yellow and darkens subjects that are violet. A No. 44 cyan filter lightens blue and blue-green and darkens light red and orange.

Correction Filters

Although panchromatic film responds to all the colors the eye can see, it does not reproduce tones of red, green, and blue objects in the same relative values as the eye sees them. The human eye is much more sensitive to green than it is to blue and red, and these colors look darker to the eye than green (fig. 3-2). Panchromatic film is more sensitive to blue and violet and looks lighter than green in a black-and-white print. This high sensitivity to blue and violet causes an overexposure to the film of blue objects as compared to green objects. This overexposure causes a dense negative image that results in a light print image (fig. 3-3).

A No. 8 yellow filter with panchromatic film helps to reproduce colors of a daylight scene with the same brightness relationship as seen by the human eye.

When using tungsten lighting, you can use a yellowish green No. 11 filter to reproduce the natural brightness relationship with panchromatic film. The yellow in the filter absorbs the ultraviolet radiation and some of the blue light, while the green in the filter absorbs some of the red light.

Figure 3-2. Color sensitivity of the average human eye.

Figure 3-3. Color sensitivity of panchromatic film.

Table 3-2. Recommended Filters for Use with Black-and-White Panchromatic Film in Daylight

To obtain desired effects with the use of filters, refer to table 3-2.