The Power of Photoshop's Lab Color and Channel Mixer

Converting Color Images to Black & White with Photoshop

Ted Robledo and I were swapping recipes for doing color to black & white conversion the other day and the first point was a consensus that just changing Mode to Grayscale sucks, as does Desaturate, or just adding a Black Solid Color adjustment layer with its transfer mode set to Color. So what else is there? Here’s Ted’s method, followed by mine.

Well, here’s Ted’s method:

RGB to B&W via Lab Color

  1. Start with an RGB image.
  2. Go under the IMAGE menu under MODE and choose LAB COLOR.
  3. Go to the CHANNELS palette-you’ll see 4 channels.
  4. Drag the B channel to the TRASH, deleting it.
  5. Now delete the channel called ALPHA 2, leaving just ALPHA 1 (which was the original lightness channel)
  6. Go under IMAGE menu under MODE and choose GRAYSCALE.
  7. Go to the LAYERS palette and click on the BACKGROUND layer. If your grayscale image appears too light, make a copy of the layer and
    change the Blend Mode to MULTIPLY. If it appears too dark, lower your MULTIPLY layer’s OPACITY until you dial in a perfect-looking grayscale

I’ve tried this on a few pics and the results are quite good. I still advocate adding an s-curve and agree that you need to play with the opacity of Multiply layer.

RGB to B&W via Channel Mixer

OK, my turn. My own B&W conversion story evolved from playing with individual channels and using a single channel’s data for the B&W image (something I read; I did not figure it out myself).
This often yields nice results and I encourage you try it (see steps below), but just as often it yields crap. The concept is sound, but we can improved it.

The solution is to choose the best mix of data from the 3 channels that give you the richest black & white data, and this varies considerable depending upon the nature of your picture. I’ll number the steps because Ted did, but really this is one step.

  1. Start with an RGB image.
  2. Go under the LAYER menu choose NEW ADJSTMENT LAYER — CHANNEL MIXER.
  3. Check the monochrome option at the bottom.
  4. Now, using the sliders and with Preview enabled, move the amounts of the red, blue, and green channels you want to add together for your final image. You should remember that the total percentage should be around 100% — if you add 70& red, 80% green, and 50% blue the image will probably be completely blown out.
  5. That’s it — except for perhaps adding an s-curve.

Experiment, see what works for you and your images.

RGB to B&W via Single Channel

This method is not recommended except as a learning experience. To begin you need to realize that Photoshop is nothing more than a very good grayscale editor underneath. To prove this look at the channels pallet. What do you see? A grayscale red channel, a grayscale green channel, and a grayscale blue channel. Hell, even masks are just grayscale data.

Knowing this it makes sense that going from a three channel RGB file to one channel grayscale means some compromises, and, generally speaking, PS does a decent job when you change color modes. But also realize that PS is doing some averaging of the three grayscales (Red, Green,
and Blue channels) to get to that one channel. And average may not suit your particular needs.

If you take a peek at, say, the blue channel you’ll most likely be surprised at what you see. The blue channel is very often much noisier than either the red or the green. And it looks rather flat, less dynamic. Conversely, look at the green channel. The
human eye is most sensitive to green (a fact that digital camera makers know well, digi-cams have twice as many green “receptors” as red or blue)
and so the green channel is often packed with clarity the others lack (again, we’re talking averages here, if you’re looking at a picture of red M&Ms, then all bets are off).

What does all this prove? Well, sometimes you just copying one channel, say the green channel, and create a new grayscale document from it yields a good black and white that’s better than averaging RGB via changing the Color Mode to Grayscale. On the other hand, sometimes this is crap. Just try this:

  1. Start with an RGB image.
  2. In the palettes change from LAYERS to CHANNELS
  3. Alt+click on the visibility (eyeball) for each channel to solo that channel’s grayscale image until you find one that you like
  4. Control+A to select the entire canvas and control+C to copy the layer
  5. Open a new grayscale image (PS will make the image the same size as what you just copied into the clipboard)
  6. Paste the image into the new file