Adobe Photoshop CS4 provides us with a great tool for getting an overview of the tonal range of an image: the Histogram panel (Window – Histogram). The histogram is basically a chart which shows how many pixels are present at each luminosity value, with dark pixels on the left and lighter pixels on the right. If no selection is made, the histogram reflects all the pixels in the image. It is also possible to select just part of an image and use the histogram to examine the distribution of pixels just within the selected area. If the image consists of multiple layers, Photoshop displays a pop-up menu which allows you to choose either “Entire Image” or “Selected Layer” to determine which pixels are included in the output shown in the histogram.
Many photographs shot in medium lighting and containing a variety of colours will have a bell-shaped curve with the most pixels in the middle of the histogram and the number of pixels gradually tailing off in the lighter area to the left and right.
The great thing about photography is that it can feature any number of subjects, be shot under different lighting conditions and have different artistic aims. Thus, for example, if an image deliberately has a lot of white: say a white sofa against a white wall, the histogram will have a peak on the left with pixels tailing off in the middle and right. In the context of this image, the skewed shape of the histogram is perfectly healthy. Similarly, if you look at the histogram of a photograph shot at night, don’t be surprised to see a very lopsided chart with most of the pixels on the right… So how do you use the histogram to spot tonal problems in an image?
Histograms are particularly good at detecting whether an image lacks contrast. Images that lack contrast have a histogram where the number of pixels tails off to zero or close to zero in the brightest and darkest parts of the image. Such images tend to have a dull, muddy appearance.
The quickest way to correct the problem of an image that lacks contrast is to use the Levels command. Choose Image – Adjustments – Levels and, in the Levels dialog, drag the right triangle over to the right to the position where the pixel distribution falls to nothing. Next, drag the left triangle which represents the white point of the image over to the right to where the pixels disappear. When you click OK, you will notice that the image has a lot more contrast and sharpness.
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