What is a Histogram in Photography?

In this walkthrough, we've explained how to read a histogram, so you gain a better understanding of an image's tonal values and improve your photography skills. 
What is a Histogram?
Essentially, a histogram is a graphic display that gives you a tonal breakdown of your image. Ranging from black to white, the histogram's horizontal axis measures tonal brightness from 0-100%. Along this tonal spectrum, you'll also find shadows, midtones, and highlights. Meanwhile, the vertical axis measures the amount of a particular tone. All of this information is visualized with a grey shape. 
Remember, there's no such thing as a "correct" histogram shape. It all depends on your light conditions and creative goals. For example, if you're trying to capture a dark image with plenty of shadows and only a small sliver of light, your histogram will be weighted more heavily on the left (i.e. dark tones). Likewise, if you're shooting a balanced city scene with a variety of tones, the histogram should have a wavy shape across the tonal spectrum. 
In short: your photographic goals should dictate how the histogram looks, and not the other way around. 
What is an RGB Histogram?
Many modern cameras also have an RGB histogram setting, which gives you a standard histogram and 3 color channel-specific histograms. This gives you a more detailed breakdown of each color, so you can see how the colors are weighted across the tonal spectrum. Your standard histogram will probably have the RGB measurements as well (represented by their color), as well as diagrams for yellow, cyan, and magenta.   
What is Clipping?
Generally, your histogram should not be touching either extreme of the tonal spectrum (i.e. completely black or white). This is called clipping, and it can reduce your image quality. If you notice this phenomenon on your histogram photography, you can adjust the exposure settings to reduce or increase your brightness before taking the shot.   
With a digital camera, if you hit total black or white in your shot, that tonal detail is lost. This means if you lighten or darken the image, the affected areas will simply turn grey. If you're trying to shoot an extremely dark or bright scene, this clipping won't be a problem, but if you do it unintentionally, your image will suffer in post-production. 

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