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A histogram is essentially a graph that shows the varying intensity of brightness as well as combined and individual colors in a specific image.
To access the histogram in Photoshop click on Window, and choose Histogram. This opens a small window with what looks like a mountain range.
The chart is currently in its simplest form where it displays all the colors in the image combined into one graph. The histogram starts at black on the left and colors get lighter and lighter to the right until you reach white. Black is 0 and white is 255. This means that if you have any pixels in the histogram touching the left or right edge, then you are either underexposing (too dark) or overexposing (too light). In this histogram you can see that the image is quite a bit underexposed (lots of pixels on the far left of the chart are at 0 black), as well as having some blown highlights (some pixels at 255 on the right side showing white).

If you refer back to the article on bit depth you will see that an 8 bit image has the ability to display exactly 256 possible values per color channel which is why there are 256 vertical bars in a histogram. The height of the histogram contains the information on the number of pixels of that color that are contained in the image.

Keep in mind that the above is a very general histogram which shows all the colors combined at the same time. We can choose to split the histogram into its composite colors in Photoshop by clicking on the small down arrow in the upper corner and selecting expanded view. This allows us to change between the RGB histogram above, or the red, green, blue, luminosity or colors view. You can also select All Channels View which will let you see all of them at once provided you have the room on your screen. It will look like this.
All Channels View
The color histograms show you the distribution of each color channel from darks to lights and how many pixels at each brightness level there are. Here we can see that all the color channels are underexposed, the blue worst of all. The composite histogram only shows us the average which is not very useful because you could have one channel overexposed and you wouldn't know it.
Most cameras only show a histogram of only one color channel, not even a combined averaged one. Some of the older Nikon DSLR's for example only showed the green channel. This was quite useless because you would never know what the other color channels were doing by judging from the in-camera histogram. Channel specific histograms are very useful to have in-camera to check your exposure.

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