product conforms closely to the desired tonal values
(based on a D-min of 0.40).
Trigradient Control Curves
As stated previously, the trigradient tone
reproduction method is based on three tone-control
curves (or printer curves): high contrast, medium
contrast, and low contrast.
Basically, the printer curves are produced the same
way that sensi-strips are made for process monitoring.
Instead of using a sensitometer to expose the film, the
contact printer is used to expose the film through a step
tablet. Since the amount of exposure is unknown, the
horizontal axis of the curve indicates the density of the
step tablet. The vertical axis represents the density
produced after the duplicating film is exposed through
the step tablet and processed.
Each set of printer curves consists of a family of
response curves that graphically display the various
tones produced in the duplicating material when it is
printed under various exposure and processing
conditions. Each curve is labeled with the exposure
setting used on the printer to create the curve (figs. 4-30,
4-31, and 4-32). These response curves enable the
density range of the imagery to be altered, so the density
range of the final product conforms closely to the
desired density range (1.00). The exposure of the
printer is important since it determines the placement of
the tonal values of the imagery being reproduced on the
sensitometric response curve of the duplicating
material. Ideally, all tones should fall on the
straight-line portion of the response curve to ensure that
the images are reproduced uniformly. The D-max of
the imagery being duplicated is used to determine the
required exposure since it reproduces as the D-min in
the reproduction.
Trigradient Tone Reproduction Procedures
The following procedures are used in the trigradient
tone reproduction system:
1. Obtain the three sets of tone control curves
(high, medium, and low contrasts) for the particular
duplicating materials being used.
2. Determine the D-max and D-min densities on
the roll of imagery to be duplicated. Remember, the
D-max and D-min are the areas in which you want to
retain detail, not necessarily the areas of highest and
lowest density. Do not use specular highlights or
completely black areas to represent the D-max and
D-min.
3. Determine the density range in the original, and
determine whether the tonal range should be increased,
retained, or decreased. Do this to obtain the desired
density range (usually 1.0) on the duplicate.
4. Based upon results of step 3, select the set of
response curves closest to the density range (high,
medium, or low).
5. On the horizontal axis, locate the density of the
step tablet that corresponds to the D-min density that
you selected. Go up from this point until you intersect
the 1.40 line and draw a tick mark.
6. On the horizontal axis, locate the density of the
step tablet that corresponds to the D-max density that
you selected. Go up from this point until you intersect
the 0.40 line and draw a tick mark.
7. Using a straightedge, select the characteristic
curve closest to your straight line. If the curves cross,
choose the curve closest to your D-max because this
density controls the exposure of the D-min on the
duplicate.
8. Expose and process the duplicate film according
to your determination in step 7.
9. Read the D-max and D-min of the duplicate
film. If these values are not within established
tolerances (0.05, for example), another duplicate must
be made. Minor adjustments in exposure or processing
may have to be made. For example, if the D-min is too
high, then less exposure is required. If the D-min is
within tolerance but the D-max is too low, then more
development is required (to raise the contrast).
The entire tone control system is based upon data
produced when the system is established. For the
system to be accurate and reliable, all of the major
variables must be controlled so they can be carried out
on a repetitive basis.
CAPTIONING AERIAL
PHOTOGRAPHY
Captioning and slating hand-held aerial
photography is as important as captioning and slating
still and motion-picture photography that is taken on the
ground. In captioning aerial photography, however, the
caption information should be expanded to include the
camera lens focal length, the altitude, and the direction
of the aircraft from which the picture was made and the
time of day the photograph was taken.
4-40

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