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
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.

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