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Photoshop levels

Control tone, contrast and overall color balance in an image

Layer > New Adjustment Layer > Levels
Or, click the Create new adjustment layer icon 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.

Tonal Range - Digital and Image

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.

The Levels Dialog Box

Levels dialog box
Input levels
Output levels

Figure 2.  Levels Dialog Box.  Mouse over, or click, the 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 an adjustment.

The Eyedropper section, shown in magenta, consists of a black Black eyedropper, gray Gray eyedropper and white White eyedropper eyedropper.

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 channel.

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.