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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.
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.
Note:
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 Properties panel, or by choosing Curve Display Options from the panel menu, then Histogram.
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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.
Histogram panel (Expanded view)
A. Channel menu B. panel menu C. Uncached Refresh button D. Cached Data Warning icon E. Statistics
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Expanded View
Displays the histogram with statistics. It also displays: controls for choosing the channel represented by the histogram, viewing options in the Histogram panel, refreshing the histogram to display uncached data, and choosing a specific layer in a multilayered document.
Compact View
Displays a histogram with no controls or statistics. The histogram represents the entire image.
All Channels View
Displays individual histograms of the channels in addition to all the options of the Expanded View. The individual histograms do not include alpha channels, spot channels, or masks.
Histogram panel with all channels displayed in color and statistics hidden
If you chose the Expanded View or All Channels View of the Histogram panel, you can choose a setting from the Channel menu. Photoshop remembers the channel setting if you switch from either Expanded View or All Channels View back to Compact View.
- Choose an individual channel to display a histogram of the channel, including color channels, alpha channels, and spot channels.
- Depending on the color mode of the image, choose RGB, CMYK, or Composite to view a composite histogram of all the channels.
- If the image is RGB or CMYK, choose Luminosity to display a histogram representing the luminance or intensity values of the composite channel.
- If the image is RGB or CMYK, choose Colors to display
a composite histogram of the individual color channels in color.
This option is the default view for RGB and CMYK images when you
first choose Expanded View or All Channels View.
In the All Channels View, choosing from the Channels menu affects only the topmost histogram in the panel.
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In the All Channels View, choose Show Channels In Color from the panel menu.
In Expanded View or All Channels View, choose an individual channel from the Channel menu and choose Show Channels In Color from the panel menu. If you switch to Compact View, the channel continues to be shown in color.
In Expanded View or All Channels View, choose Colors from the Channel menu to show a composite histogram of the channels in color. If you switch to Compact View, the composite histogram continues to be shown in color.
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To view information about a specific pixel value, place the pointer in the histogram.
To view information about a range of values, drag in the histogram to highlight the range.
Percentile
Displays the cumulative number of pixels at or below the level underneath the pointer. This value is expressed as a percentage of all the pixels in the image, from 0% at the far left to 100% at the far right.
Cache Level
Shows the current image cache used to create the histogram. When the cache level is higher than 1, the histogram is displayed faster. In this case, the histogram is derived from a representative sampling of pixels in the image (based on the magnification). The original image is cache level 1. At each level above level 1, four adjacent pixels are averaged to arrive at a single pixel value. So, each level is half the dimensions (has 1/4 the number of pixels) of the lower level. When Photoshop makes a quick approximation, it can use one of the upper levels. Click the Uncached Refresh button to redraw the histogram using the actual image layer.
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Note:
When making adjustments using the Adjustments panel, changes are automatically reflected in the Histogram panel.
Preview of histogram adjustment in the Histogram panel
A. Original histogram B. Adjusted histogram C. Shadows D. Midtones E. Highlights
When a histogram is read from a cache instead of the current state of the document, the Cached Data Warning icon 
appears in the Histogram panel. Histograms based on the image cache are displayed faster and are based on a representative sampling of pixels in the image. You can set the maximum cache level (from 2 to 8) in the Performance preference.
Note:
A higher cache level setting will increase the redraw speed for large, multi-layer files, but requires additional usage of system RAM. If RAM is limited or you work mainly with smaller images, use lower cache level settings
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To refresh the histogram so that it displays all of the pixels of the original image in its current state, do one of the following:
Double-click anywhere in the histogram.
Click the Cached Data Warning icon
.Click the Uncached Refresh button
.Choose Uncached Refresh from the Histogram panel menu.
For information about cache level, see Histogram panel overview.
You can use the Info panel to see the color value of pixels as you make color corrections. When you work with the Properties panel, the Info panel displays two sets of color values for the pixels under the pointer. The value in the left column is the original color value. The value in the right column is the color value after the adjustment is made.
You can view the color of a single
location using the Eyedropper tool 
. You
can also use up to four Color Samplers 
to
display color information for one or more locations in the image.
These samplers are saved in the image, so you can refer to them
repeatedly as you work, even if you close and reopen the image.
, and
if necessary, choose a sample size in the options bar. Point Sample
reads the value of a single pixel, other options read the average
of a pixel area.You can view color information for specific pixels in the image while adjusting color in the Properties panel.
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Make adjustments in the Properties panel. As you make adjustments, view the before and after color values in the Info panel. Move the pointer over the image to view color values at the pointer location.
Note:
If you are using a command from the Image > Adjustments menu, the Eyedropper tool
is
activated (and other tools temporarily disabled) when you move the
pointer over the image. You still have access to the scroll controls
and to the Hand and Zoom 
tools
using keyboard shortcuts.
Once you’ve added a color sampler, you can move or delete it, hide it, or change the color sampler information displayed in the Info panel.
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To move a color sampler, drag the sampler to the new location.
To delete a color sampler, drag the sampler out of the document window. Alternatively, hold down Alt (Windows) or Option (Mac OS) until the pointer becomes a scissors and click the sampler.
To delete all color samplers, click Clear in the options bar.
To delete a color sampler while an adjustment dialog box is open, hold down Alt+Shift (Windows) or Option+Shift (Mac OS), and click the sampler.
- To display or hide color sampler information in the Info panel, choose Color Samplers from the panel menu. A check mark indicates that the color sampler information is visible.
- To change the color space in which a color sampler displays
values, move the pointer onto the color sampler icon
in
the Info panel. Then, hold down the mouse button, and choose another
color space from the menu.