Understanding Histograms

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.

How to Read and Use Histograms - Digital Photography

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.

Clipping

Example of Clipping - See Left and Right Values are not at 0 brightness

Highlights or shadow clipping can occur when the brightness values in a scene, based on the exposure settings used, fall outside the histogram range. There is very little you can do post-process to recover the detail in those areas of the image. Shooting in RAW does offer some latitude for recovery, but the clipping has to be slight for there to still be detail. For this reason, it is important to view the histogram in the camera on the image preview screen and correct the shot in the field, rather than attempt to fix it later.

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.

Example of Under Exposed Photograph

Example of Under Exposed Histogram

Under Exposed

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.

Example of Correct Exposure

Example of Correct Exposure Histogram

Correctly Exposed

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.

Example of Over Exposed Photo

Example of Over Exposed Histogram

Over Exposed

Too much light has reached the sensor. Notice how the histogram reveals that the tonal distribution is pushed all the way to the right and that the highlights are clipped. If printed, the areas of the scene which should reveal detail will appear as pure white.

High Key, Low Key and Contrast

Stream feeding Talla Reservoir

High Contrast scenes create a histogram that spikes at both the shadow and the highlights ends of the scale with fewer values in the midtones area.

The road to Storr

Low Contrast scenes show the majority of brightness in the scene sits in the midtones region, with very few pixels at the highlight or shadow ends of the histogram.

Swan in Flight

If the majority of brightness values sit at the highlight end of the spectrum, the images is often referred to as being High Key.

Reflections of Glasgow By Night

When most of the values at the shadow end of the histogram is sometimes referred to as being Low Key.