Understanding histograms

The histogram: you'll find it in image editing programs and on the displays of the best digital cameras. The peaks and troughs provide information on the distribution of the brightness of an image and therefore on the quality. ifolor shows you how you can create even better photos using the histogram.

In order to understand what a histogram represents, you can try teasing out the Greek translation of the term. "Histogram" comes from "histos", meaning something like a net or tissue, and "gram", i.e. representation. This net-like representation shows the distribution of grey or colour values in an image. Using the histogram you can, with a little practice, work out the image's brightness and contrast range, i.e. whether the subject was underexposed or overexposed. Thus, you can often use the histogram to weed out a bad photo after taking your shots, even though it may actually look quite well on the screen.

In a balanced image, the peak that you see on the histogram tapers off slightly to the right and left. The dark tonal value of the image is shown on the left, while bright structures can be seen on the right. Along with the histogram, the best digital cameras, especially SLR cameras, also show the under- and overexposed areas in the image itself by flashing on to surfaces. On the histogram, however, you can clearly see which tonal value (bright or dark) is most strongly concentrated in the image.

If the image is optimally exposed, the histogram tapers off gently on both sides and will neither rise halfway up (underexposed) nor run out well before the end of the graph lines (overexposed). An image is also overexposed if there are several small peaks on the right side. If, however, there are reflections and some flashes of light in the image, then several peaks and troughs can arise in the histogram. It therefore depends on the subject and the conditions at the time the photograph was taken as to what a histogram will look like.

Along with the brightness and the contrast, the colours present while you're taking the photographs also play an important role. Since the human eye is more sensitive to green than other colours, the camera's sensor measures more green than blue and red. Due to automatic programs or exposure to strong light, colour shifts can occur in the image that are only visible on the monitor at a later stage. As the normal histogram is only of limited help here, most cameras offer a special colour histogram in order to detect this whilst taking the photographs.

Sometimes, though, you just want to play around with the brightness and contrasts in an image and use them artistically. This type of technical design for an image is called high- or low-key photography. Here, too, the histogram can help you achieve the best results – for, if the image is dark, there will be lots of pixels with low tonal values on the left side of the histogram. Bright images, however, will have lots of high tonal values on the right of the histogram.