Contrast Enhancement Via Channel Mixer

Article and Photography by Ron Bigelow

www.ronbigelow.com

Photoshop CS3 Used in this Tutorial

When it comes to enhancing contrast, Curves is probably the most popular tool. It is very versatile and fairly easy to use. However, even Curves has its limitations. When the detail in an image, or part of an image, is composed of tones that are very similar, it becomes difficult to enhance the contrast using Curves. Figure 1 shows an image that has this problem. This image was converted from a raw file using the default settings in the raw converter. No other editing has yet been performed.
Figure 1: Unedited Image
Obviously, this image needs the contrast enhanced. However, some parts of the image have detail that has very little tonal difference. This is especially true for the rock above the alcove. Figure 2 shows a close-up of this area.
Figure 2: Close up of Rock
As can be seen in the close up, the rock above the alcove has tonal differences that are very slight. Of course, the first thought would be to use Curves to improve the contrast of the detail. However, a curve that works well with the rest of the image does not improve this area very much. This is shown in Figure 3 which shows the same area with Curves applied.
Figure 3: Close up of Rock with Curves Applied
There is some improvement in the contrast, but the area needs further enhancement. A steeper curve could be used, but it would make the rest of the image too contrasty. Luckily, there is another solution that uses the channels to improve the contrast. To utilize this approach, the Channels palette is accessed (see Figure 4).
Figure 4: Channels Palette
Looking at the individual channels (see Figures 5 -- 7) shows that the red channel has very little contrast in the area of interest. The green channel has much better contrast. However, the best contrast is to be found in the blue channel. The problem is that the image takes the data from all three channels. This diminishes the detail that exists in the green and blue channels. However, a new layer can be created that takes most of its detail from the green and blue channels. This layer can be used to increase the contrast (and thus the detail) of the image.
Figure 5: Red Channel
Figure 6: Green Channel
Figure 7: Blue Channel
Figure 8: Layers Palette

The next steps start out with the Layers palette. To create the new contrast enhancing layer, the Background layer is duplicated by dragging the Background layer to the Create a new layer icon at the bottom of the Layers palette. The new layer is renamed as the Contrast layer. Figure 8 shows the Layers palette after the Contrast layer has been added.

With the Contrast layer selected, the Channel Mixer is launched by choosing Image/Adjustments/Channel Mixer. The Channel Mixer (see Figure 9) adjusts the layer by mixing the detail from the three color channels. In other words, the Channel Mixer takes a percentage of the data from the red channel, a percentage from the green channel, and a percentage from the blue channel. The important point is that the photographer gets to decide on what the mixture (i.e., the percentages) will be. For this particular image, it is necessary to increase the amount of the green and blue channels while reducing the amount of the red channel. Since this layer is designed to add contrast to the image without affecting the color balance, the Monochrome box is checked. Some experimentation is required to find the settings that create the desired contrast. The settings shown in Figure 9 create a mixture that works well for this image.

Figure 9: Channel Mixer
Figure 10: Contrast Layer
Figure 10 shows the Contrast layer. As can be seen, the contrast is significantly increased in this layer (the detail above the alcove can now be seen). At this time, it is necessary to blend this layer with the rest of the image. This is done by changing the Blend mode to Luminosity and adjusting the Opacity to a level that produces the desired results. For this image, an Opacity of 34% works well.
Figure 11: Gaussian Blur Dialogue Box
Clicking the Contrast layer on and off will reveal that, in addition to increasing the contrast, the Contrast layer has also produced a sharpening effect. This sharpening effect can be reduced or eliminated by slightly blurring the Contrast layer. This is done by choosing Filter/Blur/Gaussian Blur. The radius of the Gaussian Blur (see Figure 11) is selected to produce the desired reduction in sharpening. A radius of 2.0 worked well for this image.
Figure 12: Layers Palette After Work on Contrast Layer
Figure 12 shows the Layers palette after the work on the Contrast layer has been completed. Figures 13 -- 16 compare the original crop, the crop with a Curves adjustment (instead of the Channel Mixer), the crop with the Channel Mixer technique applied, and the crop with both the Channel Mixer technique and a final Curves adjustment.
Figure 13: Original Image
Figure 14: Image with Curves Only
Figure 15: Image with Channel Mixer Applied
Figure 16: Image with Channel Mixer and Curves Applied
It can easily be seen that the use of the Channel Mixer results in far better detail in the rock. From this point on, the image can be processed normally. Figure 17 shows the final image after some additional editing has been performed.
Figure 17: Final Image

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