Layer > New Adjustment Layer > Levels
Or, click the Create new adjustment layer icon
at the bottom of the Layers panel.
With the Adobe Photoshop Levels control, we can control tone and contrast in
three areas: highlights, shadows and middle tones. The adjusting of color
balance using Levels is discussed on the
Levels Color Correction page.
Figure 1. Histogram
We can think of the digital
tonal range as going from 0 to 255 with 0 being solid black and 255 being
solid white. We can also think of this as the lower and upper limits.
In other words, we cannot have a tone darker than solid black and we cannot have
a tone lighter than solid white.
Given any image's tonal range, it too has a maximum of solid black and solid
white. However, many image's tonal range, by their nature, do not fill the
entire digital tonal range. This is neither good nor bad, it just is.
Is there a way to tell how an image's tonal range compares to the digital tonal
range? Yes. That is what the
histogram shows. On a histogram, the digital tonal range is the X axis
with 0 on the left and 255 on the right. The black curving mass that is
plotted in the histogram is our image's tonal range. If the curving mass
covers the entire X axis, then the image's tonal range is as broad as the
digital tonal range. If the curving mass is smaller than the entire X
axis, then the image's tonal range is smaller than the digital tonal range.
Again, one is not better than the other. But this information is important when
working on our images.
The way I think of the Levels adjustment is it gives us the ability to stretch
or compress tonal ranges. It allows us to stretch our image's tonal range
to fit more, or all, of the digital tonal range. We can also compress the
digital tonal range to something smaller than 0 - 255 and subsequently force the
image's tonal range to fit within the newly defined range. No Photoshop
control allows us to stretch the digital or image tonal range beyond solid black
and solid white.
Figure 2. Levels Dialog Box. Mouse over, or
labels to see the different sections.
A color coded Levels dialog box is shown in Figure 2. As we can see,
the Levels dialog box has many different sections. There are three
sections alone that are used to control tone: the Input Levels section, the
Output Levels section and the Eyedropper section.
The Input Levels section, shown in green, consists of a histogram, three sliders
at the bottom of the histogram and three numeric text boxes above the histogram.
To make an adjustment using the Input Levels section, we can use either the
sliders or the text boxes or both. Changing one changes the other.
The Output Levels section, shown in blue, consists of two sliders beneath the
black-to-white gradient and two numeric text boxes above the gradient.
Like the Input Levels, we can use either the sliders or the text boxes to make
The Eyedropper section, shown in magenta, consists of a black
The Channel section, shown in red, allows the adjustment to affect the entire
image (channel equal to RGB), or just a single color channel, such as the Red
The Preview check box, shown in yellow, allows us to see our changes in real
time. When this box is checked, the impact of our changes can be readily
seen in the image. When this box is not checked, we will not see the
impact until we click OK. This check box is most useful when we want to
see a before/after affect while the dialog box is open. This is
accomplished by clicking the check box on and off.
The buttons, shown in cyan, allow us to apply our changes (the OK button),
Cancel the dialog box, restore the default Levels settings (press the Alt
(Option) key and the Cancel button changes to Reset), Save a particular Levels
setting (the setting is saved as a Photoshop *.alv file, not as an alpha
channel) and then to later Load the setting. Saved settings can be used in
a different image, they are not restricted to the current image. The Auto
and Options buttons are used to set up and apply automatic settings, which are
not discussed on this web site.