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Understanding Your Camera Histogram
This is the graphical representation of the brightness of your photo i.e. the tonal range. The camera's processor locates each picture element (pixel) on a horizontal scale on the histogram. Pixels of equal brightness are arranged vertically to create lines of varying heights, this means the height of the graph within a histogram is a reference to how many pixels in a scene that were recorded with the same brightness.
Lighter images move the graph to the right while darker images move the graph to the left. So having your graph concentrated on the leftmost side (0 in digital terms=pure black) means your image is likely to be under-exposed and your graph being concentrated on the rightmost side (255 in digital terms=pure white) means an over-exposure, areas that have been over-exposed will ‘blink’ when you use the histogram during the playback. This will allow you to figure out whether you need to change the exposure settings to have your graph move to the central area for a balanced exposure. In case of an over-exposure you will either be needed to slow the shutter speed, close the aperture a little bit or reduce the ISO setting and the under-exposure you do the vice-versa.
This is an important tool to give you more information about the image by analyzing it in terms of how it has been exposed. You will most likely find yourself using the histogram when you encounter high contrast in lighting which sometimes becomes tricky to get the right exposure without either over or under-exposing your subject. e.g. when you have a very bright background with the subject occupying a small area in the scene. This will help you know whether you really need to change the exposure settings.
"A "good" histogram doesn't have to stretch entirely from the left to right side to indicate proper overall exposure." Canon
So there is nothing like a perfect histogram and depending on the kind of subject you are shooting you might generate completely different histograms with the right exposure. In some instances where you have both the background and the subject are very bright, the histogram will shift more to the right but that doesn’t mean an over-exposure. The same applies when you have a dark background with a dark subject on it. The most important thing is to ensure that the graph is not only showing at the extreme far ends of the histogram because that would mean under or over-exposure
It’s not always necessary to use the histogram, most of the time you will use your eye to judge the kind of exposure you have in your image. Remember a light meter reads the scene before you take the photo; the histogram analyzes the photo you've just taken.
Primary histograms are single and are used to evaluate the brightness but there are cameras that have secondary colour histograms used for colour photography, they are three graphs that show the colour intensity of RGB (Red, Green and Blue). If you want to adjust the colours you use the white balance settings in your camera