A histogram is a graphical representation of the levels of brightness in an image. Most modern digital cameras offer a histogram display after you have taken a picture. If you are shooting on film, you can scan your photograph into a program such as Photoshop and display a histogram there (in the levels display). A typical histogram looks something like this:

In a histogram, the number of pixels of each brightness level from black (0) on the left to white (255) on the right are counted and displayed as a bar chart.

The histogram is one of the most powerful tools that digital photography offers to the photographer, because you can immediately check the exposure of a photograph after you have taken it. A properly exposed picture should have a gentle curve up at the dark end and down again at the bright end.

In this example, the ‘shadows’ - the grass and the trees - are properly exposed, represented by the left hand end of the histogram, which starts curving up nicely. But the ‘highlights’ - the brighter sky and light stone of the church - are also correctly exposed, hence the gentle curve down at the right hand end.

If a photograph is over-exposed or under-exposed, the histogram will quickly reveal this. Here is an example of over-exposure. In photography-speak, the highlights have been blown - and the detail in them is lost.

Unlike the histogram for the correctly exposed photograph, here there is no gentle curve - rather the histogram is flat with more pixels clumped up at the brightness end of the scale. And as you can see, the photograph is over-exposed, the clouds and building burnt out, with all the detail lost. If you saw this histogram on your camera display after taking a photograph, you would know that you had erred and should reduce exposure (by decreasing aperture or increasing shutter speed) and shoot again.

Of course, it is also possible to under-expose:

Here the histogram is clumped up hard against the left hand, dark end of the graph. And, as can be seen from the image, detail in the shadow areas of the photograph has been lost. Exposure needs to be increased, either by increasing the aperture or decreasing the shutter speed).

If your camera offers a histogram facility, get into the habit of checking it after each shot - especially if it is an important shot. Previewing the image on the camera’s LCD is not good enough on its own, as most camera LCDs are not large or accurate enough to enable you to judge accurately whether a shot has been correctly exposed. You should also keep an eye on the histogram when editing a photograph in Photoshop.

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