Control Your Density -
Adjust Curves
Athough you can adjust image density with a Levels adjustment quite
well, most professionals prefer to use the Curves Adjustment feature
which, as the manual states, ". . . offers the most precise control over
pixel distribution within a channel." In addition to this, you can adjust
any point along the density ramp "while keeping up to 15 other values
constant," as opposed to being able to work only with the shadow,
highlight and midtone points in Levels adjustment. If you are unfamiliar
with adjusting Levels, please see Photoshop Online Help or the Photoshop
Manual because modifying Curves is another method of image
density adjustment and the two have a lot in common.
It can be helpful to see the Histogram in the Adjust Levels window
before working on curves in order to get an easy understanding of the
density range (and those of the indivdual color channels of RGB and CMYK
images) and I find it's lack of presence in the Curves window unfortunate.
Although working with a good monitor is great, there is nothing like
seeing a mapping of the density ramp. And remember to always have the Show
Info window visible when doing density adjustments so that you can rely on
the before and after density readings (more than what you see on the
monitor).
The original scan (Figure 1) of the photo of Picture Rocks (on the
northern Michigan shore of Lake Superior) was done from a 6 x 7 (2 1/4" x
2 3/4" format) color negative on a drum scanner. Although the scanning
software has done a reasonable job in creating the positive image there
are still some density adjustments and color balancing necessary to
achieve the finalized scan below it. After experimenting with some overall
Curve adjustments, I decided to divide the sky and top-left part of the
image from the rest and do the adjustments independently on the two
sections. The area I selected with the lasso tool (using two pixels
feathering on a 300 dpi 9" x 12" file) is represented by the white section
in the mask below.
Since the bottom half of the sky was transitionally much more dense
than the top half of the sky, I added a transitional mask to my original
selection so the density adjustments I made would be much stronger in the
bottom half of the sky and gradually un-affect the top half. The final
selection mask is shown below.
After loading the selection channel shown above a simple Curves
adjustment was done to the RGB (composite) channel. The midtones,
represented by the center verticle line with an Input value of 128, were
brightened by changing it to an Output value of 170, simply by moving the
point up. Since this was the only adjustment in the Curves made, the
affected areas are closest to the midtones and there is gradual fall-off
of the affected areas so that the quartertones to highlights and the
three-quartertones to shadows are affected very little. By reading the
before and after densities on the Show Info window (our densitometer) we
see that the darkest area of the sky toward the bottom left consisted of
44% Cyan, 11% Magenta, 4% Yellow and 1% Black and is now (a much less
dense) 29% C, 4% M, 4% Y and 0% K. As we take readings toward the top of
the sky there is less and less difference between the before and after
densities because of the transitional masking of the Curves adjustment.
The final version of the sky in the bottom photo (Figure 2) shows a much brighter and
more pleasant sky after the Curves adjustment.
Now to modify the lower two-thirds of the image (represented by the
black portion of the top selection mask). Both Levels and Curves have an
Auto adjustment feature which, when used, can show the operator what needs
to be done to an image to gain maximum tonal range and accurate color
balance. I am not saying that one should rely on the Auto Curves
adjustment 100% but a lot can be learned from it's response. For the lower
two-thirds of the Picture Rocks image the Auto Curves adjustment was used
while in the RGB (composite) channel. Before hitting the "OK" button,
first see what the Auto adjustment was by going into the individual
channels. The composite channel appears unchanged but in each individual
channel the density Curve has been modified. In this case a little more
contrast was added to the rock formation which helps bring out a little
more detail. More evident is the yellow cast to the water in the
background to the horizon line has been removed and now Lake Superior has
a more attractive blue hue to it. This occurred when the quarter to
highlight tones of the Yellow channel where brightened considerably while
the quarter to midtones where much less affected and the midtones to
shadows where not affected at all. Essentially yellow has been removed
from the lighter density areas of the Yellow channel only.
This is the kind of density refinements you can make with Curve
adjustments and this also demonstrates that color balancing and modifying
can be achieved by changing the density ramps of individual color
channels. Although there are other ways to help achieve color balancing in
Photoshop, including, of course, the Color Balance window's features, many
professional scanner operators use the Curves adjustment features for both
density and color adjustments simultaneously.
Figure 1
The original scan above has too much density in the sky and a yellow
cast over the water. Curves adjustments were made as described to achieve
the finalized image below.
Figure 2
IN MEMORY OF CARL VOLK
=============================================================
>>>>Carl N. Volk<<<< April 8, 1953 - September 5,
2000
Carl's art, talents and teachings will live on for all
of us to enjoy.
Visit: http://www.carlvolk.com
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