What's the purpose of a histogram, and how would I use it?

Question: What's the purpose of a histogram, and how would I use it!?
Best Answer - Chosen by Asker:

You've got some pretty good answers above, but here's a nice reference that shows several images and explains their histograms:
http://www!.luminous-landscape!.com/tutori!.!.!.

Here's a little more concise explanation:
http://www!.dpreview!.com/learn/!?/Glossary!.!.!.Www@QuestionHome@Com

Here is a very simple explanation:

A histogram is a visual representation of all of the colors in your image!.
The left end is the darks, the right end is the lights!. Picture it as a graph where the number of pixels of each color are stacked up in the appropriate row!. In 8-bit color, there are 256 possible colors on the graph!. 0 is true black, 255 would be true white!.

The histogram is most helpful in digital photography because you can see if your chart from a specific image runs up into the whites, or down into the blacks!. If so, no detail can be recovered from those areas!.

Most cameras (and photoshop) are also equipped with the ability to see WHERE in the image you have blown out your highlights or shadows!. (Blown out meaning gone into true white or true black, with no recoverable detail!.) In photoshop, the little arrows at the top right and left of the histogram show you this!.

In can get a little complicated to explain, but that pretty much covers the basics!. The histogram allows you to keep your exposure within acceptable boundaries to keep detail in your whites and blacks!.Www@QuestionHome@Com

The histogram on the camera shows you the distribution of tonal ranges in your image!. It graphs number of pixels versus brightness, with bright being on the right!. If the graph runs off the right end, you have a lot of pixels that are pure white, indicating blown out highlights!. If it runs off the left end, you have a lot of pure black, meaning you have some blocked up shadows!. On the other hand, a significant blank space at the right shows no whites, and the picture might look a little dingy!. If not extreme, this is easy to fix in Photoshop or Gimp by adjusting the levels!. Blown highlights and blocked shadows, on the other hand, are unrecoverable!.

You'll notice the levels control shows you the same histogram!. The usual goal here is to make the images levels just fit, though if you want to have a certain amount of pure white you can drag the upper slider down past the end of the curve; same thing with black at the other end!.Www@QuestionHome@Com

Which one!?

The one you see when using Photoshop!? You can use it when using the levels tool to set the black threshold, white threshold and make small adjustments in the mid-tone range of your image

The one that shows up on DSLR's can be helpful, but can be confusing as well if you are shooting high or low key studio shots!.Www@QuestionHome@Com