Tutorial: Using the Channel Mixer for B&W Conversion

[Trevor Ash]

Part 1 of 2

Photo 1: The original image.

If you've never gone to the channels palette now is the time to start becoming familiar with it. Every RGB image is comprised of three channels of information. The red channel, the green channel, and the blue channel. For an 8 bit image each separate color channel is represented as an intensity value from 0 to 255. When you look at each color channel separately in the channel palette window it's showing you how much of that color is at each part in the image by representing it as a lightness value from 0 to 255. There's actually an option somewhere in Photoshop that lets you display it in the color of the channel instead of levels of grey but that's not the best method for most.

When looking at the RGB channel it just represents the composite of all three channels.

When we look at each channel individually we start to see which channels have "Interesting" information in them. For example let's look at the red channel...

Photo 2: This is the red channel. Notice how bright the center of the flower is. That means that there's a lot of red in that part of the image probably in the range of 200 and greater (remember we're looking at levels from 0 which means no red to 255 which means the max amount of red possible).

Photo 3: This is the green channel. Notice how dark the ladybug shell became. This means that there is much less green than red in this area of the photo. Due to the middle grey tones in the rest of the image you can conclude that there is an average amount of green (probably around 120 or so) in the rest of the photo.

Photo 4: This is the blue channel. Notice how extremely dark the center of the flower became. The ladybug shell also became very dark. This means that there is little to no blue in those parts of the photo. The flower petals have some blue in them, probably due to reflections from the sky or just color casts from other soruces.

Hopefully I've now taught you something about channels. I'll post the part about actually using the channel mixer next.

[Trevor Ash]

Part 2 of 2

When reviewing the separate channels. I think that the red and green channels contain the most "interesting" details and information. The blue channel doesn't have much of any interesting detail so I don't really care about that channel at all.

I'm going to select the composite RGB channel and then go back to the layers palette so that I'm looking at the full color image again. What I want to do it use the red and green channels to make my B&W image and completely ignore the blue channel. How did I arrive at this decision you ask? Well, like I said before, they seem to have the most relevant and interesting details for this particular photo. It could be that your photo has better detail in the blue channel and nothing of interest in the other channels. Often, for skin tones the red channel alone makes a great B&W image. Experiment a little with channels and you'll get kind of excited about them once you understand how to use them.

Now let's look at using the channel mixer to get the results I want. I want to mix the red and green channels, while leaving the blue channel out, and make a B&W image as a result.

Go to Image/Adjustments/Channel Mixer and you'll see the window shown in Photo 1 here.

The first thing you'll need to do is check the "Monochrome" option at the bottom of the dialog. This is what will make it B&W when we're done. It should be obvious to you that the three sliders labeled Red, Green, and Blue adjust the amount of each channel that you want to use for the output. If you want to subtract color information from a channel then set the percentage value for that channel to something negative. Setting a negative value will darken the image very quickly so it's not something you usually do. Values are usually at zero or something higher.

The trick to understanding this dialog and the values to enter are to adjust all channel the sliders so that they all add up to 100%. So if you want half red, half green, and no blue you'd set red to 50%, green to 50% and blue to 0%. So that's what I'm going to use as my starting point as shown in Photo 2. Make sure you keep the preview option checked so you can see your results as you adjust.

Now there's no reason you can't make the channel values add up to more of less than 100%. As long as you keep an eye on making sure you don't blow out any details you can do whatever you want. Having the values add up to 100% is just the recommended starting point. So, when I look at my mix I made of 50%, 50%, and 0% I'm not really happy with the result because it looks too dark and muddy. I'd like to brighten up the petals a little bit. When I look back at the blue channel I see that it had a lot of data in the petals and little anywhere else. This means that I can add a little of the blue channel to lighten the petals and it won't affect the rest of the image much at all. So Photo 3 shows the result of adding a little blue to the mix. Notice that the values add up to more than 100% now which is perfectly okay.

I'd also like to lighten the center of the flower a little more as well to further separate the ladybug from it. When looking back at the channels I remember that both the green and red channels contained a lot of information in the center flower portion. After experimenting with adjusting the red and green channels I find a good mix of values (60% for red and green) that lightens the center flower portion but doesn't affect the ladybug too much.

Unfortunately, the values I entered for the red and green channels have now totally blown out a lot o highlights. That's probably because my values are now R=60%, G=60%, and B=15% which adds up to 135%. This is too much for this image to handle.

Fortunately, the blown out areas are completely in the flower petal areas alone. I'm hoping by now you can guess what the solution to this problem is!!! Did you remember that the blue channel contained mostly flower petal details and very little anywhere else? Well, the solution is to just back down the blue channel again until we recover the blown highlights from the petals!

The final photo shows both the values I ended up using for the channel mixer as well as the final result. I hope I've helped you learn something today

Note: I had to sacrifice a little detail to get the tonal ranges I was after. With a little extra work I could have confined my changes so they wouldn't have blown out the highlights as quickly but still have the tones I wanted.

[Trevor Ash]

Here's some quick comparisons to the automated methods Desature, and Greyscale.

The greyscale mode actually gives pretty good results but choosing Image/Desaturate gives horrible results here!

[Trevor Ash]

Here's a photo that was done using Steve's method. What I found was that just the luminosity channel alone when looking at the LAB conversion produced a great B&W. After further following his instructions the results became worse and worse until I realized that the step about picking a grey value from the color picker is a little more critical than he had emphasized. It turns out a picked a pretty dark grey and for this image it ended up darkening it too much.

Now, here's the really cool part about Steve's method he showed us. Do you remember in my earlier examples I mentioned that with a little extra work I could have controlled the highlights from blowing out? Well Steve's method automatically controlled them!!! That part where you control click to make a selection from the grey channel and then multiple the color swatch layer is what does it.

I wil be sure to remember the lessons learned from Steve's example and will use it myself. Thanks for pointing it out Steve

[bingo pajama]

man, trevor- this is really amazing! i just want to point out again how incredibly cool this site is- where else online can you learn ps skills like this so concisely, so informative... for free?!
i'll be utilizing this knowledge from now on- just wanted to say 'thanks' to you (and steve too) for making stuff like this available to us peons.

[Asylum Steve]

Trevor, I'm glad you took the time to go into such detail explaining the color channel mixer method of bw conversion. I know a lot of people (myself included) often need the visuals to go with the explanation to better grasp a new photoshop technique.

But what I think is really cool is that you not only got a decent result with COLOR DESATURATION, but didn't aurtomatically get a good result using my LAB method.

What that tells me is that different types of colors shots: different subjects or different lighting or different tonality MAY need to be converted using DIFFERENT techniques to get the best bw results.

So, in this sense it's smart to learn several of these techniques, and don't be afraid (or in my case too lazy) to take the time to compare the results using different methods.

[danag42]

I've been using the Channel Mixer to convert to black and white for some time now, but Steve's comment about using different methods for different types of photos got me thinking: We used to choose the particular film emulsion for a job depending on the desired results. Seems that it would be a good idea to create seperate Actions for specific effects, using the Channel Mixer or whatever method you think is best,

I plan to experiment with different combinations of the different channels, to see if I can come up with a specific Action for a specific type of effect. That was an excellent leap of knowledge, Steve. That's what I like about these kinds of forums, there is a cross-fertilization of ideas that wouldn't happen in individuals working alone.