Converting color images to black and white in Photoshop

In a recent forum discussion, the question was asked what’s the best way to convert color images to black and white using Photoshop. As with almost any task in Photoshop, there are a number of ways to accomplish it and while some are personal preference, some methods do offer advantages to creating better images.

First, I’m going to define the “best” way as the method that allows you to fine tune the image the most. While some of the methods are easier and quicker than others, they provide little control over the final image.

There are 5 methods that I’m going to review and I’m going to present them in the order of least desirable to most desirable. They are:

  1. Hue/Saturation Adjustment Layer
  2. Image>Mode>Grayscale
  3. Gradient Map Adjustment Layer
  4. Channel Mixer Adjustment Layer
  5. Black & White Adjustment Layer

There are two other methods that I’m not going to cover. First is making the conversion to the Raw file before it gets into Photoshop. Converting the raw file can produce better results in some cases, but the Black & White Adjustment in Adobe’s Camera Raw works similarly to the one in Photoshop. The second alternative is using a plugin which might give you even more options. But since I am happy with the amount of control that the Black and White Adjustment Layer provides, I haven’t explored purchasing a conversion plugin.

To help demonstrate the results of each of the 5 methods, I first created a sample file made up of 7 gradients. The first three gradients go from black to one of the primary transmissive colors (red, green, blue) and then to white. The next three gradients go from black to one of the primary reflective colors (yellow, cyan, magenta) and then to white. The last gradient goes from black to white using shades of gray.

Then I used the different conversion methods to see what effect they had.

Hue/Saturation Adjustment Layer

As you can see, reducing the saturation treats every color the same. You’re simply getting rid of the color. There are no options for this method. The black and white gradient is a bit different, and that might be due to the way I created it.

Image>Mode>Grayscale

I was actually surprised by the result of this method because I expected it to be identical to reducing the saturation to zero. Especially when you see the warning message that Photoshop throws up.

It sounds like it’s going to do the same thing that the saturation adjustment would do. But this method uses some algorithm that treats colors differently. This is just a setting that some Adobe engineer decided looked good for most images. It might look good for your image, then again it might not. But like the saturation method, you have no control over the result.

Gradient Map Adjustment Layer

Using a Gradient Map to convert to black and white is a method that I wasn’t familiar with until reading about it in the forum discussion mentioned earlier. So I looked it up on the web and found an article on how to do it but the author wasn’t sure exactly what it was doing. I don’t know either. It does provide a different result than the prior two methods. It also offers one level of control. You can change the softness value in the gradient to get slightly different results. Here are two conversions. The first one was converted with a softness value of 100% and the second was a softness value of 33% which the article recommended.

Softness set at 100%

Softness set to 33%

The effect of using a gradient map is pretty close to that of the grayscale method, except that it seems to darken the blue channel more. The effect of changing the softness level is subtle at best. So even though the method has some control, it’s not related to anything that you’re trying to accomplish in the image. Which gets me to the next method.

Channel Mixer Adjustment Layer

33% Red, 33% Green, 33% Blue

Photographers who have actually shot black and white film know that the best tools to improve the photo are contrast filters, a set of colored filters. By restricting the colors that hit the film, the photographer can change the contrast of the image. Since a black and white image can only convey information from contrast, this is very important. The channel mixer method allows you to use some of the control that contrast filters do for film. The advantage that digital has over film though is that the decision of which filter to use can be made later. You can even use one “filter” on the sky, and another “filter” on the foliage, something totally impossible in the film world.

For example if you want to lighten the leaves on the trees so they stand out better, you would increase the green channel. The example above shows all three channels being set to 33%. This gives a result identical to the saturation method.

But the wonderful thing is each of the three channels can be tuned from -100% to +100%. When I was using Photoshop CS and before, I used the channel mixing method and as a starting point for most images I used settings of Red 60%, Green 40%, and Blue 0%. This difference is illustrated in the image below.

I’m not saying those are the best values to use. But for the camera that I was using at the time, and the type of images that I take, that seemed to be a good starting point.

Black & White Adjustment Layer

When I upgraded to Photoshop CS3, I found there was a new conversion method available under Adjustment Layers called Black and White. Similar to the Channel Mixer, it allows you control the mix of colors, but instead of just the transmissive primaries available in Channel Mixer, it also has sliders for the three reflective primaries. I now use this method for my black and white conversions. Most of the time I make adjustments to the red and yellow channels. But once in awhile one of the other channels becomes really useful. For example if you’re trying to make a purple shirt show up better, sliding the magenta higher can help. The six controls allow you to target individual colors better.

This image shows the output of the sample file with the settings at each slider set at 50%. This gives you the same output as the reduce saturation method.

Here is the difference when you change the magenta to -200% and the cyan to +300%.

Finally, here is the output using the default values of Red 40, Yellow 60, Green 40, Cyan 60, Blue 20, Magenta 80.

Conclusion

While all of the methods will get you black and white image, if you want to control how the final image looks, the Black and White Adjustment Layer provides the most control, and I think it is also the most intuitive. Remember also, that in addition to the color conversion, your black and white image may need further changes to the contrast, either globally or locally. Global changes can be made with a Contrast Adjustment Layer. I usually darken the shadows and sometimes I lighten the highlights as well. Local changes can be done through dodging and burning. Like color conversion, there are numerous ways you can dodge and burn in Photoshop. That’s an entirely different subject to explore.

Here is a summary then of the different methods.

  1. Hue/Saturation Adjustment Layer – Just throws the color info out. No user input. Every color adjusted equally.
  2. Image>Mode>Grayscale – Color info removed, but Photoshop applies it’s own algorithm of what it thinks is best. No user input.
  3. Gradient Map Adjustment Layer – It does something similar to Method #2, but not sure what it does. Minimal non intuitive control
  4. Channel Mixer Adjustment Layer – 3 channels x 200 levels = 8,000,000 options and intuitive control interface
  5. Black & White Adjustment Layer 6 channels x 500 levels = 15,625,000,0000,0000,000 options and intuitive control interface

If you’ve used method 5 and have also set the global and local contrast of your image, but you’re still not happy with your black and white image, then maybe it’s an image that just isn’t going to work as a black and white. Maybe it needed different lighting and/or composition to begin with to make it an effective subject in black and white.