What is a Digital Camera Histogram Display?

Many digital cameras have the ability to provide a histogram display.  Histogram tools are also provided in some software image editing applications such as Photoshop or Paint Shop Pro.

A histogram is a chart which shows graphically the distribution of pixel quantities in an image relative to brightness or luminosity.

The horizontal axis shows brightness values of 0 to 256 corresponding to the dynamic range of the camera, from dark on the left to bright on the right.

The vertical axis shows the quantity of pixels present in the image at each of the 256 horizontal brightness levels. The higher the display at a given point, the more pixels of that brightness are present in the image.

The display does NOT represent the physical position of pixels on the horizontal and vertical axes of an image.

The Camera Histogram Display

The histogram display in a digital camera is a powerful aid for assessing exposure. It serves as an effective exposure meter providing more information than an analog type light meter which gives an average value.  The histogram graphically shows the light values and their distribution over the entire dynamic range of the camera. 

Interpreting the histogram

The first basic concept to realize is that if a larger proportion of pixels is found towards the left side of the display, then the image will be relatively dark or low key. Similarly, if a larger proportion of pixels is found towards the right side of the display, then the image will be relatively bright or high key. Of particular importance in photography is how the image is exposed. The histogram is a great tool for alerting us to over and under exposure.

Over exposure

The relevant region to check for over exposure is the right hand end of the histogram display. If the chart shows a lot of pixels at this end, particularly if they are hard up to the 256 brightness level, then it is probable that the picture will have parts of it which are overexposed. Over exposure can be recognized when the picture appears to be too bright overall, lacks dark and shadowy areas and has areas which appear as burnt out or as formless flat white regions where all detail is lost. Depending on the nature of the picture content, it might be acceptable to over expose highlights if they are very small in area but they could look objectionable if they of of any appreciable size. Watch out for bright objects in your picture such as fluffy white clouds, sparkles on water, sunlight catching on buildings or a model's hair for instance.

Notice in this picture, taken near Byron Bay in Australia, the sky, clouds and sea are burnt out near the top of the photo on the left side. On the adjacent histogram graph, the over exposure is represented by the congregation of bright pixels crammed up against the right hand end of the graph.

Under Exposure

The left hand end of the histogram display is the place to look for under exposure. If the concentration is crushed over to the 0 brightness level at the left, then it is probable that the picture will have areas which are under exposed. Under exposure can be recognized when details in dark shadowy areas are can't be discerned and there are areas of formless black.

This interior picture was taken inside a Queensland house, intensionally without flash because the photographer wanted to capture the soft mood of the dim light and to exclude unwanted detail. It does however serve to illustrate under exposure which is evidenced by the grouping of dark pixels at the extreme left of the graph.

Correct Exposure

It is often recommended that a picture should be 'exposed to the right', meaning that as much light as possible, without over exposing the image, should be aimed for. The reason is to realize the maximum available dynamic range of the camera image sensor. The result will be a higher contrast image. Digital cameras inherently generate a certain amount of unwanted electronic noise which can sometimes be visible as an unwanted feint multi colored peppery or snowy random pattern in dark under exposed pictures. A better signal to noise ratio is likely to be achieved with an maximized exposure.

However, between over exposure and under exposure, a correct exposure is also subject to the personal judgment and preference of the photographer.

Breaking the Rules

There are many so called rules often quoted for various techniques and methods in photography but many photographers will agree that these can be circumvented in certain circumstances to achieve a particular mood or effect. For instance, an image could intensionally be under exposed to achieve a silhouette effect or over exposed to white out particular unwanted detail.

Taken at Neutral Bay in Sydney Harbour and shot into the afternoon sun, this high contrast picture, is an example of the photographer's preference for the image to be over exposed on the sparkling, wind chopped water for a dramatic effect. The narrow column of bright pixels on the extreme right of the graph are the over exposed high lights.

A silhouette effect is produced here. The histogram graph shows no crushing or under exposure at the dark pixel end at the left and similarly no over exposure at the bright pixel end at the right.

A low contrast picture of clouds is shown here illustrating that an image can be devoid of dark pixels. There are actually some bright pixels present but only in relatively low proportions as indicated by the very low representations on the graph extending towards the right hand end. The contrast of the picture could be increased by software editing but the soft quality of the clouds would be lost.