Photography Tips: Using Histograms

In this installment of our photography tips section, we will cover simple but often over looked tool; the Historgram.

One of the key elements which distinguishes a great photograph is a full tonal range - that is a picture which has all tones from totally black to completely white. If you analyse images which seem to lack impact, you will often find that they appear "washed out" or "dreary". What this really means is that they lack either fully dark tones or fully light ones.

Digital cameras provide a vitally important tool for displaying the tonal range of each picture in the form of a graph, called a Histogram. Unfortunately, because histograms look a 'bit technical' many people are frightened to use them. In reality they couldn't be simpler; the graph goes from black to white along the x (horizontal) axis and shows the amount of each tone by the height of the graph.

When examining a histogram there are two things you need to look for, the first is balance.

Balance

These examples show histograms which are

1. too dark - you can see the graph is showing that all of the tones in this picture are dark. In fact there are no tones above about 60% grey! You would need to increase the exposure to correct this.
2. the exposure here is correct - the graph is centred
3. too bright - the bulk of tones in this picture are light. You would need to decrease the exposure to correct this.

Range

If you look carefully at histogram (b), you will see that there are gaps at either end of the graph. This is showing you that the image does not contain any true blacks or whites. Though this is the best exposure you can obtain for a particular scene, the resulting picture will appear a bit flat and lack punch compared with the ideal shown below - where the graph covers the entire range.

If at all possible you should try to extend the range of light in the picture; if you are inside then you can change your position, or move the light sources. If you are outside then try moving round so that you and the model are facing in a different direction.

Computer Correction

Fortunately the same histograms are available in image editing packages, and it is a straight-forward procedure to make the correction - most easily with an "Auto-levels" adjustment.

For related photography tips see Exposure, ISO and the Lighting Workshop.