Adobe Photoshop CS4 - Manual (Page 160)

153 Last updated 1/10/2010
Chapter 7: Color and tonal adjustments
Photoshop® CS4 provides a comprehensive set of tools for making color and tonal corrections and adjustments.
Viewing histograms and pixel values
About histograms
A histogram illustrates how pixels in an image are distributed by graphing the number of pixels at each color intensity
level. The histogram shows detail in the shadows (shown in the left part of the histogram), midtones (shown in the
middle), and highlights (shown in the right part) A histogram can help you determine whether an image has enough
detail to make a good correction.
The histogram also gives a quick picture of the tonal range of the image, or the image key type. A low-key image has
detail concentrated in the shadows. A high-key image has detail concentrated in the highlights. And, an average-key
image has detail concentrated in the midtones. An image with full tonal range has some pixels in all areas. Identifying
the tonal range helps determine appropriate tonal corrections.
How to read a histogram
A. Overexposed photo B. Properly exposed photo with full tonality C. Underexposed photo
The Histogram panel offers many options for viewing tonal and color information about an image. By default, the
histogram displays the tonal range of the entire image. To display histogram data for a portion of the image, first select
that portion.
You can view an image histogram as an overlay in the Curves dialog box by selecting the histogram option under
Curve Display Options, and in the Curves Adjustment panel by choosing Curve Display Options from the panel menu,
then Histogram.
Histogram panel overview
❖ Choose Window > Histogram or click the Histogram tab to open the Histogram panel. By default, the Histogram
panel opens in Compact View with no controls or statistics, but you can adjust the view.
A B C