Understanding Light and Filters 319
for improving pictures, and in most cases do not penetrate haze and do not improve distant
shots. Consider polarizing fi lters for this purpose as explained later in this chapter.
Neutral Density Filters
Neutral density fi lters, also called gray fi lters, are made from neutral gray colored glass that
does not change colors or the tonal rendition. They are used in black and white and color
photography to reduce the amount of light reaching the image plane. You may want to use
these fi lters outdoors so you can photograph at a larger aperture, which reduces depth-of-
fi eld or photograph at a longer shutter speed to blur moving subjects such as water.
Neutral density fi lters come in various densities, requiring the exposure increases shown
in Table 17-1. A fi lter that requires a two f stop increase is a good choice for many applica-
tions. The fi lters can be combined for further light reduction.
Table 17-1 Neutral Density Filters
Density Percentage of light Increase in exposure
transmission in f stops
0.3 50 1
0.6 25 2
0.9 12 3
1.2 6 4
1.5 3 5
1.8 1.5 6
Filters for Black and White Photography
In black and white photography, the various colors are recorded in a range of gray tones
from white to black. Filters can be used for purposely darkening or lightening certain colors
to emphasize, suppress, separate, increase, or reduce the image contrast. You can lighten or
darken any color (see Figure 17-1). To lighten the grey tone of a color, use a fi lter of the same
color or at least from the same side of the color wheel (see Figure 17-2). To darken the gray
tone of a color, select the color directly across the wheel or at least from the opposite side. A
yellow fi lter darkens colors from blue-green to violet. Green fi lters lighten green and darken
purple and red; they can be used to lighten shaded green areas under trees (see Figure 17-3).
In black and white photography, a yellow fi lter produces some improvement in distant shots;
orange and red fi lters can do more. Polarizing fi lters are most effective in digital photography or
with fi lm but only if the light comes from the right direction, as explained in the section on polar-
izing fi lters. Complete or almost complete haze penetration can be obtained with a red fi lter com-
bined with infrared fi lm, but other colors are also changed; for instance, green appears as white.
PARTIAL FILTERING
It may be desirable to fi lter only part of a scene, such as darkening a bright sky to bring it
closer to the brightness level of the landscape below or adding a touch of blue to a white
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