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The histogram represents the tonal qualities of your image: in other words, it shows you how light/dark your image is.
Imo, it's a very important tool that is generally under-used, probably because it is not fully understood.
Firstly, it's important to understand that there is no 'right' or 'wrong' histogram, as different images will have different tonal ranges. But it IS valuable to be able to look at the histogram for a particular image and understand what it is showing you.
When you look at your histogram, you see a graph with various peaks and troughs. The left hand side of the graph represents the darkest tones and the right hand side represents the brightest tones. Obviously, the middle shows you the mid-tones. A peak occurs when there is a large number of pixels recorded at that level of brightness. Note that it's unimportant what colour those pixels are.
If the graph touches either the left or right hand side, this means that information has been 'clipped', or lost. If it touches the right hand side, your image is overexposed in at least one place and highlight clipping has occurred so that portion of your image will display as pure white, with no detail. A graph touching the left hand side means that parts of the image are underexposed and shadow clipping has occurred - portions of the image will display as pure black without any detail.
In general, clipping in either shadows or highlights is not a good thing. However, there are times when it's acceptable (even desirable) - for example, when shooting against a black background, you would expect to have a large peak at the left hand side of the graph. You want the background to appear as pure black, without detail, so that's fine.
It's always a good idea to keep referring to your histogram as you process your image, as any edits that you make will alter the tonal range of the image. If at any point the shadows or highlights are pushed to the edge of the graph and clipped, it's usually advisable to either mask out some portions of the image or to undo the edit altogether.
This histogram shows an image with a narrow tonal range, low contrast and mostly comprised of midtones. Neither shadows nor highlights have been clipped:
This histogram shows an image with a much wider tonal range. Some highlight clipping has occurred and, if you look at the image, you can see that this portion of the graph represents the blown out window, in the top right corner. When a portion of an image is blown, it is important to be able to recognise and identify the blown area and then decide whether or not it is detracting from the image as a whole:
This histogram represents an image with very high contrast: there is a lot of information at the shadows side and a lot of information at the highlights side, with very little in between. The shadows have been clipped, and are showing as pure black. There are also clipped highlights:
Once again, there is no right or wrong histogram. It is up to the individual to decide whether or not clipped shadows and highlights are acceptable in the context of the overall image. The important thing is being able to identify them in the first place
Last edited by Wintersong; October 26th, 2009 at 12:18 AM.