How to read a histogram: what shadows and highlights tell you
The little screen on the back of your camera lies. In bright sun every photo looks too dark, in a dim room too light. Yet many photographers base their exposure on that screen. There is a better gauge, and it often sits right next to your photo: the histogram. Learn to read it and you know at a glance whether your exposure is right, without the surrounding light fooling you.
What a histogram shows
A histogram is a graph of all the brightness values in your photo. From left to right it runs from deep black to pure white. The height at each point tells you how many pixels have that brightness. A peak on the left means many dark areas, a peak on the right many light ones. It is not a judgment of right or wrong, it is a count. But that count tells you exactly where the light in your photo sits.
Left, right and what sits between
The left side is your shadows, the right your highlights, and the middle the midtones, where most skin tones and details live. A well exposed photo usually has a hill somewhere in the middle with room on both sides. But that is no law. A dark, moody photo leans left, a light, airy photo leans right. The shape follows your subject, not the other way around.
Clipping: where you lose information
It gets interesting at the edges. If the graph sticks to the left edge, your shadows have closed up to pure black and there is no detail left to recover. If it sticks to the right edge, your highlights have burned out to white. That is called clipping. A little is often fine, a deep shadow may be black. But a blown sky or a scorched white shirt will not come back afterwards. The histogram warns you before it is too late.
There is no perfect histogram
A common myth is that a good photo has a tidy hill in the middle. That is not true. Photograph a black cat on a dark sofa and your histogram should sit on the left. Photograph snow and it should sit on the right. Force it to the middle and your cat turns gray and your snow dirty white. The histogram is a tool, not a rule. Read it in the context of what you are shooting.
Exposing to the right, in moderation
There is one rule of thumb that often helps: expose as bright as you can without burning your highlights. You push the graph as far right as possible. The reason is technical: your sensor records more information in the light tones than in the dark ones, so there is less noise there. When editing you bring the photo back down and keep cleaner shadows. Do not overdo it, because the moment your highlights clip you lose information instead.
Learn to read it yourself
You grasp a histogram fastest by putting a few of your own photos beside one. With the histogram reader you upload a photo and instantly see how shadows, midtones and highlights are spread, with an explanation per zone. Your photo stays in your browser. Try it once with a successful photo and once with a failed one, and you will see right away where the difference lies. After that you will never look at that little graph on your camera the same way again.