Black-and-White Conversion Using the Channel Mixer
We generally use the Channel Mixer (as an adjustment layer) to convert the individual color channels in an image to grayscale values. The most important settings to make are:
Activate the Monochrome option (Figure 10-2). The sliders are then automatically set to +40, +40, and +20. These are the default conversion values.
Then use the individual RGB sliders to determine how much each channel contributes to the resulting grayscale image. A good rule of thumb is to set the values so that they total 100%. More than 100% results in a brighter image, and less makes the result darker. As of CS4, Photoshop automatically displays the total at bottom right of the dialog window.
Check the individual channels ...