Nikon LS-3500 - LS-3510 - Manual (Page 66)

7-2 Scanning for Reproduction
Software Reference for Scanners
up the highlights, would make the shadows overly green. The two extremes must
be corrected separately, so we use the gamma curve controls of our image
processing application to neutralize (or add magenta to) the greenish shadows and
remove or subtract the magenta in the highlights. If the highlights are already too
bright, we can add green to them (making them darker and neutralizing the
magenta at the same time). If the midtones are being affected, then we can use the
gamma controls to adjust the middle balances.
This midtone control is essential for balancing skin tones with greater
accuracy while preserving the balance in the light and dark areas of the photo. In
any case, usually some combination of these techniques can be applied to improve
bad color casts and “muddy” images. You will need practice to determine the
range of control movement for a particular effect. In general, small changes in the
gamma curves are very noticeable, and a subtle approach will yield more pleasing
results. The beginning technique, however, involves making more extreme
changes and then choosing some point between these extremes.
This applies to using all the Analog Controls. For example, when adjusting
the Blue balance to correct an overly “cold” or bluish image, move the control all
the way from 50 to 35 and see what happens. If the resulting image is too “warm”
or reddish-yellow, then you can estimate the proper correction much more quickly
than if you had only corrected to 48 and still found the image too “cold”. It is
always better to go beyond the anticipated correction first, and roll it back later to
fine tune the results.
Color Reproduction Background Information
The color dyes that are used in photographic film and papers are formed when
color couplers, compounds that are essentially colorless in their unprocessed state,
are developed out with tanning developers to form a color dye with certain spectral
characteristics.
To begin with, color films contain the same light-sensitive silver halide grains
which are found in black and white emulsions. The couplers are con-contained in
separate silver halide layers with a colored filter layer between each. These filters
will pass only certain bands of the spectrum to the emulsion layer below. The red
sensitive layer receives red light, the green layer, green light etc. After exposure
and development, there exists a fully formed negative metallic silver image
representing the image in each layer, R, G, and B.
For a slide film, the latent (unprocessed) silver image is first developed to
form a metallic silver where the silver halide (usually bromide or chloride) has
been exposed to light, then the silver image is chemically reversed from negative
to positive, and then the couplers that are attached to each silver crystal are further
developed to form a color dye corresponding to the metallic silver grain at that
location. The dyes that result are cyan at the red silver grains, magenta at the green
silver grains, and yellow at the blue silver grains. After this is completed, the silver
is bleached away and the dyes remain to form the color image.