a slight increase in density. This is called the
threshold and indicates the least amount of exposure
that will produce a noticeable change in density.
Straight-Line Section
Further to the right (fig. 2-6) is a section of the
curve that appears to be a straight line. Note that in
many cases there will not be a well-defined straight
line. This section of the curve has a constant slope,
and in addition, the gradient of the slope here is
greater than in any other part of the curve. In the
straight-line section of the characteristic curve, there
is an equal increase in density for an equal increase in
exposure. This is the most important part of the
curve. Subject exposures that fall on the straight line
produce constant density differences.
Shoulder Section
The upper section of the curve (fig. 2-6) is a
convex, curved line that gradually decreases in slope.
This section of the curve is called the shoulder. Like
the toe area, equal changes in exposure do not
produce equal differences in density. Tones of the
subject falling very far up in this section are blocked;
that is, reproduced with densities too heavy for
printing or maximum detail. For normal exposures,
bright highlight tones of the subject tend to be
reproduced in the shoulder section of the curve.
Exposure
Although sensitometry is a tool of the lab
technician, it also is significant to the photographer.
Notice how this applies to exposing film.
For
example, when a uniformly lighted gray card is
photographed, there is a single exposure,
corresponding to a single point somewhere on the
log-H axis. When the light on the surface of the gray
card is increased and another photograph is made
(maintaining the same camera settings), the exposure
and the log H also increase. This causes a shift to the
right on the log-H axis.
There should be a
corresponding increase in density, and the two factors
again should plot on the characteristic curve.
Extending this to a scene with a large number of
luminances (reflectances) or high-luminance ratio, the
tonal differences in the subject, the lighting, and the
camera settings determine the film exposure that
produces varying amounts of densities. These density
differences must be related to the log-H differences
that produce them; that is, density differences in the
negative must be considered in their relationship with
the tones of the subject.
The density differences in a negative can be
partially controlled by placing the exposures
corresponding to the subject tones in the correct
section of the characteristic curve. This is done by
adjusting the camera settings correctly, providing that
the range of tones in the subject is not too great.
Emulsion Latitude
The emulsion latitude is the exposure range where
there is a proportional relationship between density
differences and log-H differences. In other words, it
is simply the range of exposure covered by the
straight-line section of the characteristic curve (fig.
2-6). The latitude of an emulsion, therefore,
determines the brightness range of the subject that can
be reproduced proportionally. Latitude may be
expressed either as the difference in log-H values
between the two extreme points of the straight line
or as the exposure ratio between these same two
points.
The emulsion latitude of light-sensitive materials
varies according to the purpose for which it was
designed—from 1:400 or more for long-scale
panchromatic film, and from 1:20 or less for process
film. For any given emulsion, the emulsion latitude
varies according to the contrast. The emulsion
latitude decreases as the contrast increases.
Log Exposure Range
The useful exposure range includes part of both
extremities (toe and shoulder) as well as the
straight-line section of the curve. For ground pictorial
film the useful exposure range of a sensitized material
is greater than the emulsion latitude, since portions of
the toe and shoulder regions of the curve are used
without sacrificing print quality.
The approximate lower limit of the useful
exposure range
is the density point on the
characteristic curve that is not less than 0.10 above the
gross fog of the film. This point is referred to as
minimum useful density. The upper limit is generally
located at 90 percent of maximum density on the
shoulder of the curve and is referred to as maximum
useful density. In practice, many photographers use a
2-12
