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By Matt Spinelli, 4/8/2006
In part 2 of this user guide we began to talk
about the photo editing aspect of s7raw, namely color balance and exposure
adjustments. We will continue this section by going over the Adjust2 tab that
features tone curves, contrast, brightness, saturation, and hue.
Making Adjustments - Tone Curves, Contrast, Brightness, Saturation, and
Hue
Tone Curves
The gamma and log control affects the tone curve of an image. Tone curves are
not as easy to master, or explain, as other controls. Nevertheless, I'll try to
make it as easy as possible to understand. Please note though, that s7raw has
an even better control that allows for more customization called the free tone
curve found on the curve tab that allows for more advanced and precise
adjustments. We will discuss this in a later tutorial. In this section, I will
focus on how the gamma and log control affects the tone curve.
Moving the slider to the left when using either gamma or log will cause the
image's histogram to shift and condense to darker tones. Sliding to the right
will cause the histogram to shift and condense to brighter tones. Subtle
adjustments are useful for bringing out (or hiding) detail in primarily the
shadows and mid-tones of the image. Extreme adjustments will alter the entire
image to become either bright or dark. Some examples of the effects are seen
below.
Very dark tone curve
Very bright tone curve
Tone curves do not cause any clipping to occur to the image as can happen
when using sensitization (exposure compensation), contrast, brightness,
saturation, and hue. Clipping is when the values in the histogram go below the
darkest value possible or above the brightest value possible. In other words,
highlight detail or shadow detail is lost if clipping occurs. Tone curve
adjustments work within these boundaries to prevent clipping due to the
logarithmic nature of the control.
Lastly, most editors do not have a log adjustment but instead have only gamma
that is generally used for image correction. After contacting the makers of
s7raw, I found out that Fuji raw files (RAF) have their data stored in what
appears to be a logarithmic curve. Thus s7raw kindly allows us to adjust our
raw images in like manor. Gamma adjustments are similar to log, but
mathematically they are not the same. You can see the difference in switching
between the two. Which one is best? Which ever looks right to you!
Contrast
Simply, contrast is the difference between the darkest and lightest tones in
the image. Increasing contrast results in a heightened difference between dark
and bright areas of an image. Decreasing contrast will reduce this difference.
Unlike the white balance and exposure compensation adjustments, contrast
expands or condenses the brightness and color values away from or toward the
midpoint of the histogram depending on which way the slider is moved.
For example, using the test image, drastically decreasing the contrast will
compress the tones towards the middle of the histogram. This will produce a
grayish effect as is shown below.
Decreased contrast
If we were to drastically increase contrast it will expand the tones away
from the histogram. This will produce a very bright and dark image as is shown
below.
Increased contrast
Obviously, the goal under normal circumstances would not be to produce
images with extreme contrast or the lack there off. Nevertheless, this
illustrates quite well how the contrast control affects the image.
Brightness
Brightness behaves in a similar fashion to that of exposure compensation in
that it too will affect the overall brightness or darkness of the image as a
whole. However, unlike exposure compensation, which expands or condenses the
image's brightness levels and colors, brightness adjustments simply shift the
histogram values to the left (darker) or right (brighter) as is shown by the
two examples below.
Decreased brightness
Increased brightness
Saturation
Saturation is best described as the intensity of color. More saturation will
bring out and emphasize colors, giving an image more vibrancy. In extreme
applications it will bring out color noise and cause severe color channel
clipping. Less saturation will cause an image to be more subdued, and in
extreme cases will convert an image to become grayscale.
With s7raw, saturation can only be adjusted to the image as a whole, and
unfortunately we have no control over individual colors. High saturation may
bring out a color we want to accentuate, but as a result less desirable colors
too will be emphasized since we don't have control over individual colors. As
such, I suggest that saturation be applied only mildly when using s7raw, and
instead use Photoshop or some other photo editor to perform saturation on
separate colors. However, since raw images do not have any in camera processing
applied to them (this includes saturation), modest use of this tool normally is
necessary. Just leave the fine tuning to another editor.
Looking at the histogram, it is interesting to see how saturation adjusts our
test image. I have created an animated GIF file to show what happens. I started
with the saturation control at -100 (all the way to the left). Then I adjusted
it to -50, 0, 50, 100, 200, and 300 (all the way to the right). Saturation
appears to amplify the difference between the primary red, green, and blue
channels. Visually, the dominate colors, and in this case red, will become the
strongest when more saturation is applied. Notice too that some clipping in the
darker values occurs (as seen by the blue and green channels on the left side
of the histogram).
Animation of Saturation
Hue
Hue shifts the color of an image. Moving the hue control to the left will
introduce a more violet or blue tone. Shifting the hue control to the right
will introduce a more yellow or green tone. The exception is that shifting to
either extreme will produce near identical results. Basically having a hue
value of -100 or 100 is essentially the same.
The histogram isn't as useful in using the hue control as it is in the previous
ones, but it will tell you if you are clipping any of the color channels. If
the hue values are at -100 or 100 it will show that the color channels have
swapped places compared to when it is set to 0.
In s7raw, the hue control itself shows what happens when it is adjusted. This
too I have animated. There are two color strips below the control. The top one
shows the colors before a hue change. The bottom strip shows what the
corresponding color in the strip above will become after the hue is
changed.
Animation of Hue
Adjust2 Tab Tutorial
After discussing the two main adjustment tabs in s7raw, it can become a little
confusing to know in what order to make these adjustments. While not a hard
fast rule, here is a possible order of adjustments to make to give pleasing
results:
1) White balance (Adjust1 tab)
2) Sensitization (Adjust1 tab)
3) Saturation and Hue
4) Contrast and Brightness
5) Gamma or Log
Using the sample image and the settings discussed in the previous article (I used example #1) we are going to
add contrast, brightness, saturation, hue, and log adjustments to the sample
image. These adjustments will be much more subtle than the above examples. The
before and after of the image is shown below, as well as the parameters used.
The results are not very dramatic as the image is pretty good as is. If you
look closely you'll see more detail in the darker tones of the leaf area, more
vibrance in the colors, and slightly less contrast.
Adjust 2 parameters
Before Adjust2 adjustments
After Adjust2 adjustments
Lastly, I've mentioned a lot about using the histogram. The reason is
that it will help us understand what is happening and when we might be clipping
any of the red, green, or blue channels. However, do not forget to look at the
image as the final determining factor in whether or not to apply an adjustment.
Oftentimes there is more than one way to achieve the same, or similar, result
by using a combination of the adjustment tools. In our next article we will
discuss the detail tab.
Continue to part 2c - making adjustments with
the Detail tab
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