Dodging and Burning

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These terms describe techniques used in the darkroom to adjust the brightness range when printing negatives. Natural scenes have a much greater span of brightness than can be captured by film, which in turn is greater than can be reproduced in prints. Whereas the human brain can see detail in shadows even under bright sunlight, much of the detail in shadows is easily lost when a photographic print is made.

The techniques of 'dodging and burning' were used when printing negatives, to recover lost detail in areas of the photograph which were considered to be important. A large part of the skill involved in making photographic prints was related to applying the right amount of correction to the right parts of the print. The American photographer Ansel Adams was one of the key proponents of the technique, which is described in his book The Print (Adams 1995)

'Dodging' refers to holding a piece of card in the beam of an enlarger to 'hold back' exposure of parts of a print that are not wanted to be too dark. Conversely, burning refers to applying extra exposure to parts of the print that are to be darkened.

Similar limitations in displaying the range of brightness in a natural scene arise when processing digital images. The sensors in modern cameras have a very large dynamic range (the span from the smallest amount of light that can be detected to the brightest). Unfortunately, it is impossible to display the whole of this range on a display device, such as a computer screen or an ink-jet print.

So, the old darkroom techniques of dodging and burning are still applicable when processing digital image files. Figure 1, below, shows an approximately linear gradation from dark (RGB = 0,0,0) to fully bright (RGB = 255,255,255). The 8-bit system used by computer monitors allows just 256 values for each of the red, green, and blue components of each pixel in the image, meaning that 256 x 256 x 256 different colours can be displayed (that is 16,777,216 colours).

Fig. 1 Uniform Gradient from (0,0,0) to (255,255,255)

Although the RGB values increase uniformly across the image, most of the 'action' appears between values of about 40 and 150. The image shows very little visible variation below 40, where there may be a lot of shadow detail in a photographic image.

Extracting the detail in this region of a photograph is not straightforward. If we increase the brightness (in a program such as Photoshop) by 40, this merely increases all the values across the whole image, and the brighter areas are 'burnt out', since a value greater than 255 cannot be displayed. The result is shown in Figure 2, where it can be seen that a chunk of the overall brightness range has been 'lost'.

Fig. 2 Effect of increasing Brightness by +40

The technique of 'dodging and burning' applies a different range of values to different parts of the image, depending on the original brightness. For example, if we select all those parts of the image with a brightness below 100 (RGB = 100,100,100) and then adjust the brightness and contrast of these parts of the image separately, we can achieve the range shown in Figure 3,below:

Fig. 3 Effect of Splitting the Image into Different regions

To demonstrate how this technique works on a 'real' image, I have chosen a photograph of a Magpie (Pica pica). This bird illustrates the difficulty of maintaining detail in both the white and the black areas of the plumage. The original jpeg image is shown in Figure 4, below.

Fig. 4 Original Magpie Image

Notice that there is feather detail in the white areas,which we don't want to lose, and very little visible detail in the black areas around the head. The first task is to select the white areas, to protect these from being burnt out by further processing. The selection can be made with the lasso tool or by using the 'magic wand' tool with appropriate parameters. After making the selection, use the 'Inverse' command in the Select menu to transfer the selection to all parts of the image except the white. It is a good idea to feather the selection by about 5 pixels, to prevent a hard line at the edge. The results of the selection process are shown in Figure 5, below.

Fig. 5 Selection of the Whole Image Except the White Plumage
(dotted lines surround the selection area)

Now that the white areas are protected, the brightness and contrast of the the rest of the image can be adjusted. In this case, I used the levels control to brighten the foliage and the head area of the magpie

Fig. 6 Adjustment of Non-white Areas of the Image

Detailed adjustment can now be made with the Dodge and Burn tools. I used Photoshop Elements (PSE) v.2, where these tools are readily available. They are still available in later PSE versions, but only as options within the toolbox.

The 'Dodge' tool appears as a pin symbol. I set it to work on 'Highlight' areas of the image with a value of about 8%. I then work the brush of the tool (shown as a white circle) over areas that I want to lighten - in this case, the head and base of the bill and also, to a lesser extent, the back plumage. The process is shown in Figure 7, below.

Fig. 7 Using the Dodge Tool

To control the tool effectively, it is a good idea to zoom into the area which is being worked upon. Lightening an area also tends to reduce the colour saturation, because of the interaction between different Photoshop tools. I therefore use the Saturation Sponge tool to restore the colour, with a setting of about 7% flow, as shown in Figure 8, below:

Fig. 8 Using the Saturation Sponge

The final result is shown in Figure 9. The overall effect is subtle - my aim is not to distort the image but to achieve a result which represents the perception of the original scene.



Fig. 9 Result of Dodging and Burning

Mike Flemming, Feb. 2011

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