the straight line intersects the scale. As you can see,
the gamma for the plot is 0.82.
CONTRAST
A negative contains several different contrasts. It
has "total contrast," defined as the difference in
density between the useful shadow and highlight
densities in a negative. Total negative contrast is a
useful index to determine what contrast printing filter
to use. Total negative contrast is dependent upon
factors, such as subject luminance ratio, camera
exposure level, color of light, and gamma. Gamma is
only one of the factors that controls the total contrast
of a negative. In addition, a negative has other forms
of contrast as follows:
Shadow contrast--associated with the toe
section of the characteristic curve and,
therefore, unrelated to gamma.
Highlight contrast--associated with the
shoulder of the characteristic curve and
gamma does not apply.
Midtone contrast--associated with the
straight-line section of the characteristic curve.
GAMMA
Gamma, therefore, is not a measure of negative
contrast, nor does controlling the gamma in different
negatives ensure that they will all print with the same
contrast printing filter. All of the frames on a roll of
film may be developed uniformly to the same gamma;
for example, 0.75. However, each negative probably
has a different total contrast because of variations in
subject luminance ratio, subject color, lighting, and so
forth. "Contrast," therefore, is best defined as a range
of densities produced by a combination of the subject
luminance ratio and the amount of development given
a film.
Film exposed on a cloudy bright (no shadows) or
heavy-overcast day could be developed to a high
gamma (for example, 1.6) and still produce a flat
negative, because of the small luminance range of the
subject being photographed. Another scene with a
high-luminance ratio could be photographed and
developed to a low gamma of say 0.50, yet the
contrast of the negative could be so high that it
requires a low-number contrast printing filter.
You should understand that even though a
negative can have high-total contrast, there may be
little contrast in the shadows when those shadows fall
on the flat portion of the toe. When two films with
the same characteristics are exposed to the same
scene, at the same time, and each film is developed to
a different gamma, more contrast can be expected in
the negative developed to the higher gamma. This is
true for those tones exposed on the straight-line
section of the characteristic curve and to a lesser
extent for tones exposed on the upperpart of the toe.
When the basic formula for determining gamma is
examined,
D (difference in density) may be
considered the negative contrast (for the straight line)
and
log H (difference in log H) the subject contrast.
Gamma can then be considered as the ratio between
negative and subject contrast. A negative that is
developed to a gamma of 1.00 has, for all straight-line
exposures, the same contrast range as the original
scene. When the negative is developed to a lower
gamma (for example, 0.50), it has only half as much
contrast as the subject. Remember, this applies only
to the straight-line section of the curve.
Gamma, however, is not always appropriate for
measuring the effects of exposure and development.
Gamma does not take into consideration that the toe
of the curve is normally used for recording shadow
tones in ground pictorial, continuous-tone film. Also,
D-log H curves for different films have different toe
lengths and toe shapes; consequently, film developed
to a given gamma may not yield a uniform density
range sufficient enough for ordinary continuous-tone
photography. To provide a more uniform density
range, you can use a form of averaging the gradient,
called contrast index. However, in some applications
where the characteristic curve has a long straight-line
region and the image is recorded totally on the
straight-line section of the curve, gamma is still a
valid method of measuring density range.
In aerial photography, for example, it is desirable
to record shadows, midtones, and highlights on the
straight-line section of the characteristic curve. When
all subject tones are recorded on the straight-line
section of the curve, the greatest amount of tone
separation is obtained in all areas of the image
(shadows, midtones, and highlights). This provides
the maximum amount of detail in all areas of the
negative. A greater emphasis is placed on detail,
rather than a "pretty picture," in most aerial
photographic applications.
2-16

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