How to Use the Histogram Panel in Photoshop CS6
In Photoshop CS6, a histogram panel displays the tonal range (also referred to as the key type) of an image. It shows how the pixels are distributed by graphing the number of pixels at each of the 256 brightness levels in an image. On this graph, pixels with the same brightness level are stacked in bars along a vertical axis.
The higher the line from this axis, the greater the number of pixels at that brightness level. You can view the distribution for each color channel separately or for the composite image as a whole.
From this graph, you can then determine whether the image contains enough detail in the shadow, midtone, and highlight areas. This information helps you determine what image adjustments you may need to make.
Choose Window→Histogram to bring up this graphical wonder.
By default, the histogram displays the tonal range of the whole image, in the composite image’s color mode, such as RGB, CMYK, Grayscale, and so on.
Select Compact View, Expanded View, or All Channels View from the Histogram panel pop-up menu:
Compact View: The default. Displays only a histogram of the whole image (or your chosen selection or channel) with no controls or statistics.
Expanded View: Shows a histogram with statistics and controls for selecting and viewing the histogram of individual channels. This view also has controls for refreshing the histogram to show uncached data and choosing a selected layer.
All Channels View: This view displays all the options of the Expanded View, plus the individual histograms for each color channel. You may also choose to view your channels in color.
Choose a different source in the Channel and/or Source pop-up menu, if needed.
For example, instead of seeing a histogram for an entire image, you can display the histogram of an individual channel, alpha channel, or spot channel from the Channel pop-up menu. You can also focus on the selected layer or an adjustment layer. Just select the layer in the Layers panel and choose Selected Layer or Adjustment Composite from the Source pop-up menu.
If the Cached Data Warning icon (a triangle with an exclamation mark) appears in the upper-right corner of the histogram, click the Uncached Refresh button just above the icon to see a histogram that reflects the image’s current state.
The warning lets you know that Photoshop is reading the histogram from cache rather than showing your image’s current state. Cache is a reserved, high-speed section of your computer’s memory. The image cache allows the histogram to display faster because it’s calculating the histogram based on a representative sampling of the pixels in your image.
With the Histogram panel displaying the controls and data you want to check, examine the tonal range in the histogram.
An image with good tonal range displays pixels in all areas. An image with poor tonal range has gaps in the histogram.
If you’re into numbers, check the statistics to evaluate your image, as well.
Position your cursor within the histogram to see statistics about a specific value. Drag your cursor within the histogram to see statistics about a range of values.
When you make adjustments based on problems you see in the histogram, be sure to select any Preview options in the dialog boxes of your image adjustments, such as Levels. That way, the Histogram panel displays both the original and adjusted histograms.