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Customary Method - RGB channel

This method is one of the easiest and is non-destructive.  We simply look at the red, green and blue channels individually on the Channels panel.  Find the one that we like the most and leave that channel as the only active channel and we are done.

The disadvantage of this method is the information stored in the other two channels is never used in the calculation of tone.  For example, if an area of an image has the RGB values of 15, 208, 237.  The tone on the Curves control would be (15*.30) + (208*.59) + (237*.11) for a total of 153.  153 is lighter than mid tone.  If we use the Red, Green, Blue Channel method and choose the red channel, the resulting tone for this area of the image would be 15, since that is the red channel value.  Tone 15 is much darker than mid tone.  Another disadvantage is we lose the ability to control tone by color.

Figure 1 shows the red channel of the two sample images.  Figure 2 shows the sample images converted to retain Measured luminance.

Red channel Figure 1.  Red Channel

Measure luminance Figure 2.  Preserve Measured Luminance