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Back in Black: from color to grayscale > Using the Lightness Channel

Using the Lightness Channel

This method of converting an RGB image to grayscale lets you isolate just the luminosity in the photo, separating out the color; and by doing so, you often end up with a pretty good grayscale image. However, since this uses the Lightness channel, we also add one little twist that lets you “dial in” a perfect grayscale photo almost every time.

Step One.
Open the color photo that you want to convert to grayscale using the Lightness method.


Step Two.
Go under the Image menu, under Mode, and choose Lab Color to convert your RGB photo into Lab Color mode. You won’t see a visual difference between the RGB photo and the Lab Color photo—the difference is in the channels that make up your color photo (as you’ll see in a moment).

Step Three.
Go to the Channels palette and you’ll see that your photo is no longer made up of a Red, a Green and a Blue channel (as shown far left). Instead, the luminosity (the Lightness channel) has been separated from the color data, which now resides in two channels named “a” and “b” (shown at right).

Regular RGB image.

Converted to Lab Color.

Step Four.
We’re interested in the grayscale image that appears in the Lightness channel, so click on the Lightness channel in the Channels palette to make it active. (Your photo now looks grayscale onscreen too, as it displays the current active channel).

Step Five.
Now, go under the Image menu, under Mode, and choose Grayscale. Photoshop asks if you want to discard the other channels. Click OK.

Step Six.
If you look in the Channels palette, you see just a Gray channel (as shown here).

Step Seven.
Go to the Layers palette, click on the Background layer, then press Command-J (PC: Control-J) to duplicate the Background layer. Switch the Blend Mode of this duplicated layer from Normal to Multiply, and you see the photo become much darker onscreen.

Step Eight.
Chances are, changing that top layer to Multiply made your photo too dark (as shown here). Since you’re using a “multiplier” effect (the Multiply Blend Mode), getting a darker result is fairly common, but you fix that in the next step.

Step Nine.
This is where you get to “dial in” your ideal tone (and fix that “too dark” look from the Multiply layer). Just lower the Opacity of your Multiply layer in the Layers palette until you have the tonal balance you’ve been looking for. Below, you see the final conversion from color to grayscale, using the Lab color method, which gives you much more control, and depth, than just choosing Grayscale mode from the Image menu.

Standard grayscale conversion.

Lab Lightness Channel conversion.



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