In the last article
about scanning (N&T, June 2001), I provided a quick overview of histograms
and tone curves.
Before moving on to a new area, tone curves
deserve a little more attention. While they are easy enough to manipulate,
getting them right can be tricky. There are two ways you can make tone
corrections — the linear method or the non-linear method.
One of the problems with either type of tone
correction is that some of the gray level information is eliminated in order to
make the specified adjustments. It either compresses or expands the selected
area. This happens each time you correct the tone curve. Once you apply the
correction and save the file, the information is gone forever. This is also why
I always work on a copy of the original when I am making any corrections after
scanning.
The experts say that the best time to make
corrections is during the scanning process when you have the most information,
but, of course, that’s easy to say when you’re an expert. For the rest of
us, let’s just say it is best to avoid correcting over corrections.
Linear tone corrections
As the name suggests, a linear correction
modifies the image in a flat, straight line across the entire image. I think of
linear corrections as either placing a veil over an image to darken it or
peeling away a layer to lighten it.
Look at the illustrations. You will see that
darkening an image causes the 45-degree linear tone curve to be shifted to the
right, or white end of the input axis. Shadow detail is completely lost because
values are clipped to black. No white or bright highlights will be present in
the image now, meaning that overall contrast has been reduced.
Brightening the image moves the tone curve to the left, or black end of the
input axis, sacrificing all highlight detail by clipping it to white. Black and
dark shadows are eliminated, resulting in a smaller tonal range.
Reducing overall contrast rotates the tone curve
in the opposite direction, compressing the full input range into only the output
midtones, removing light and dark tones.
There are times when an overall effect is
desirable — on a badly over-exposed original, for instance.
Non-linear tone corrections
Modifying different points in a tone curve is
known as non-linear correction. Gamma correction is another term for non-linear
tone corrections.
Image-manipulation programs use different methods
to edit gamma or tone curves. Some offer a freehand drawing tool, which is
difficult to control precisely, although smoothing may be possible. Others allow
the curve to be split into a number of control points, which can be moved to new
positions manually, or in some cases numerically. Some have sliders.
Tone corrections should always be made before
color correcting. With some scanners, you can save tone curves for use with
other, similar images. Now you can apply that tone curve during scanning. This
way the software can make use of the extra, oversampled data on corrections that
require expansion of tonal ranges, such as shadow areas.
Some tips for non-linear corrections
You improve a dark original by lifting the
quarter tones and midtones. This increases the shadow details and brightens the
image. Because highlight details are virtually absent, compressing this area is
not a problem.
Lowering the shadow three-quarter tone and
raising the highlight quarter tone produces an S-shaped curve that will give a
low-contrast image more snap. Midtone contrast and detail is increased, and
although highlight and shadow details are compressed, they are not lost
entirely.
A high-contrast image containing few midtones may
be improved by lifting the shadow quarter tone and dropping the highlight
quarter tone, expanding these areas into the midtone output range.
Rosemarie Monaco is the chief executive
officer of Group M Inc., a marketing communications and consulting firm
specializing in the graphic arts. Send comments and questions to rmonaco@groupm.org.
