Tonal Range adjustments
You can use the Curves or Levels effects to adjust the tonal range
of an image. Here are some basic rules about adjusting tonal range:
- Ranges are divided into highlights, midtones, and shadows
- A bitmap with full tonal range should have about the same amount
of pixels in all the three ranges
- Images with a concentration of pixels in the highlights looks
washed out
- Too many pixels in the midtones makes an image appear bland
- A concentration of pixels in the shadows makes the image lose
a lot of detail
The Levels filter sets the darkest pixels as black and the
lightest pixels as white and then redistributes the midtones accordingly.
This creates an image with the most detail in all of its ranges.
The Histogram in the dialog box shows the distribution of the pixels
in the image between shadows (left) and highlights (right).
The photo above has obviously a high concentration of pixels in the
shadow region. The picture looks dark and not much detail is visible.
I applied the levels effect and used the automatic adjustment. The
result of the adjustment looks like this:
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You can tell that the pixels
are distributed more naturally.
Of course, since the subject of the photo is a dark butterfly
in front of dark background there is no way to really change
the appearance of the entire image. As an image to work with
this one is a good example for a difficult image.
Below you will see the new Histogram after the automatic
redistribution of the pixels in the image. Notice how the
solid spike in the dark tones has changed. |
 You
can also make manual adjustments by using the eye dropper tools
to select your own values for shadows, midtones, and highlights.
Another way to adjust the appearance is to use the sliders under
the histogram or to limit the range of the output levels. Doing
it manually however is very difficult and takes a lot of patience
and familiarity with image editing.
The Levels effect also allows for individual adjustments of one
of the three color ranges, Red, Green, or Blue.
You can use the Curves filter to achieve the same adjustments
we just managed to create with the Levels filter. Only the graphical
rendition of the values is different. Instead of the histogram the
Curves effect serves the purpose of graphically adjusting any point
in the color range, not just highlights, midtones, and shadows.
It is more precise but also takes more practice to control.
The horizontal axis depicts the original brightness of the pixels
(input) while the vertical axis shows the new value (output). Since
in the above screenshot there were no changes made the input and
output are always the same.
 Let's
look what happened to the Red channel when the adjustment is made.
You can see that there is a distinct shift towards the brighter
range, the input now starts at 22 instead of 0. That means that
22 is the darkest red pixels, not 0.
The picture is adjusted in the same way you have seen it above
- by the levels automatic adjustment.
In the Curves effect you can grab any point of the line and drag
it to make specific values darker or lighter. When the Preview checkbox
is checked you can see the effects right on the screen. The eyedropper
tools serve the same function as in the Levels effect.
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