Controlling contrast: channel mixing

We’ve looked at how to optimise image contrast using the Levels and Curves dialogs, but these tools can only enhance what contrast is already contained in the image. The problem with black and white photography is that contrasting colours can often end up with similar tonal values, and neither the Levels nor Curves dialogs can help you much in such instances. Precise contrast management is a two-step process. The first step is to manage the colour contrasts – and that’s where the Channel Mixer comes in. You use it to adjust the contrast of the three colour channels (red, green and blue) to exaggerate the tonal contrasts between the colours. It sounds technical, but it’s a lot easier than you might imagine.

Mix it up

The Channel Mixer dialog (Image > Adjustments > Channel Mixer) has three sliders, one each for the Red, Green and Blue colour channels. The first thing you need to do is click the Monochrome box; this sets the output channel to grey. Next, make sure the Preview box is checked (it should be by default) – you’ll need to be able to see the results of your changes ‘live’ as you make them. You can move all three colour sliders independently, but aim for a combined value of around 100% in order to achieve brightness levels roughly similar to those of the original image. The Constant slider adds or subtracts a constant value from all three colour channels; you’re unlikely to need to move it from its default 0% setting. The Red slider is set to 100% by default, and the other two to 0% – this gives you an instant red filter effect.

BEFORE-We chose this image because it contains a wide range of rich, saturated colours that will illustrate the effects of different Channel Mixer settings very clearly.

RED-Here we’ve simulated a deep red filter by setting the Red channel to 100% and the other two to zero. Watch out for image degradation with compact digicam images, which often have noisy red channels.

YELLOW-We’ve produced a subtler result here, simulating a yellow filter by combining Red and Green (50% of each). This still darkens the blue sky but it also lightens the greens, which is useful for editing landscape shots.

GREEN-Here we’ve simulated a green filter by setting the Green channel to 100%. Note how the green and yellow parts of the train are now similar in tone, and compare this with the effects of the red effect.