It's just a simple graph, but the histogram has had a major impact on how we go about exposing our photographs on location and processing them later.
Histograms have a vertical axis showing the number of pixels in each of the 256 brightness value channels, and the horizontal axis represents the brightness of the light. The latter ranges from pure black on the left to pure white on the right. This range of brightness is often split into the broader categories of shadows, midtones and highlights. How the histogram looks relies entirely on the scene it's representing, so there is no one, ideal histogram shape to aim for.
When exposing for landscapes, there are two rules you really need to think about. The first is to expose for a midtone and the second is to avoid burnout.
A midtone is an area of the scene that falls halfway between the darkest and lightest parts of the shot. Once located, you can use the midtones as a reference point to ensure that the midtones in the scene translate to midtones in the captured image. It's a simple concept but ensures that the foreground details are clearly visible for a natural looking picture. Grass and other foliage make a reliable midtone, so long as it's under the same lighting conditions as the majority of the shot.
Burnout occurs when highlight tones become so bright they turn pure white - all underlying detail is irrevocably lost. A sport of burnout around the sun is perfectly ok, but huge stretches of a boring white sky are not. Ideally, you want all the major parts of the scene to contain plenty of colour and detail.
Avoiding burnout in the sky, while getting a good foreground exposure, is easier said than done. The reason being that the vast tonal range of the landscape is often too much for the camera to handle. it can only capture a slice of what's on offer, so something has to give. If you expose for the midtones then there is a good chance the sky will burn out. If you expose for the highlights, then the foreground will fall into silhouette. You need to think carefully about what's most important in your scene and expose accordingly. of course, if you don't want to compromise you can use a neutral density or graduated filter.
How to Read a Histogram
You can call up the histogram on your camera's LCD to judge the tonal distribution in a photo. The horizontal axis show pixel brightness, ranging from pure black on the left to pure white on the right. The vertical axis shows the number of pixels at a particular brightness level.
Not enough light has reached the sensor, either too fast a shutter speed or too narrow an aperture, or both. This shot's histogram is pushed all the way to the left, a situation known as clipping. If printed, the clipped areas will appear completely black and all shadow detail will be lost.
This histogram should correspond with the tones of your scene. A dark scene should have a histogram with a bell shape on the left. A light scene should have a histogram with a bell shape on the right. For daylight scenes, expose the scene as far to the right as possible without clipping the highlights.