It sounds simple: just take the color out of your image and you have done black and white digital photo processing.

Maybe it is the power of suggestion from current events, but this past year I have been fascinated by black and white photography. I’ve read many posts and watched many YouTube videos. Black and white photography has many, many varieties and nuances. It could be the study of a life time.

In this post I am going to talk a bit about what I learned, not as an expert, but as someone who might be able to make things more accessible.

As always, I have focused on how to use open source software. There are four software programs I have used: Raw Therapee, The Gnu Image Manipulation Program (a.k.a. the GIMP) and two plug-ins for the GIMP: G’mic and Nik Silver Effects (an older free version).

Shooting mode: imagination

One of my first takeaways: Digital photography is natively in color. To maximize the number of artistic options available to you, you are better off not using the monochrome options built into you camera. Except: If you use a camera that shoots, and is set to save, both jpg and raw files this isn’t a big problem, because the raw file (which is just the data from the sensors) will be in color. Because of this you have to use your imagination to take pictures you plan to convert.

In black and white you are relying on composition elements like layout (rule of thirds, golden spiral, etc.), leading lines, shapes, etc to draw the eye to the focus point you want. Often in black and white contrast takes the place of color to draw the eye. While in photography contrast is usually defined as the difference between light and dark, there can also be contrast between textures and shapes, and between areas that are in sharp and soft focus.

Another thing to keep in mind, is that some tools allow you to select which colors are dark and light, so contrast in color can be created, even if they are the same tone in the original. For example you can darken a sky to make clouds pop, or lighten (or darken) the color of a flower petal to make an insect more visible.

Converting to Black and White

The first method of turning a photo into black and white is “simple” de-saturation. Except that it isn’t that simple. For a digital image the color image has three monochromatic channels: red, green and blue. They are created by the demosaicing algorithm that translates the camera sensor data into an image. If you shoot in JPG then the camera does this, for raw files you need to use a demosaicing algorithm (I use Raw Therapee most of the time). These channels combine to create the color image. You can, if you wish, create a black and white by simply de-saturating one of the channels.

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But wait…

Black and white digital photo processing isn’t actually that simple. The different channels tend to contain different types of information, as the example above shows. The article Perfect black and white with GIMP’s mono-mixer by Mora Photo describes this (worth reading even if you don’t use the GIMP, since most programs have a channel mixer and it describes the characteristics of each channel with examples). Because each channel has different characteristics, almost all de-saturation methods de-saturate all three channels, then recombine the channels. Different percentages for different effects. Most programs have a tool where you can specify the percentages for each channel.

The most simplistic de-saturation method is a combination of equal parts from each channel. More sophisticated methods try to create a sense of how your eye sees light and dark, for example, we tend to see a pure red as darker than a pure green. There are various formulas to account for that. Most are approximately 30% Red 60% Green and 10% Blue.

You can approximate the effect of different old school film types, by using different mixes of the de-saturated channels:

Film type	Red Percent	Green Percent	Blue Percent
Agfa 200X	18	41	41
Agfapan 25	25	39	36
Agfapan 100	21	40	39
Agfapan 400	20	41	39
Ilford Delta 100	21	42	37
Ilford Delta 400	22	42	36
Ilford Delta 400 Pro & 3200	31	36	33
Ilford FP4	28	41	31
Ilford HP5	23	37	40
Ilford Pan F	33	36	31
Ilford SFX	36	31	33
Ilford XP2 Super	21	42	37
Kodak Tmax 100	24	37	39
Kodak Tmax 400	27	36	37
Kodak Tri-X	25	35	40

The table above illustrates how complex a “simple” black and white photo can be.

How to find the channel mixer for each program:

Raw Therapee has a “channel mixer” method option in the Black and White tool under the colors tab. Raw Therapee also has tools to selectively change the brightness of individual colors.

The GIMP has a simple Mono-mixer: Colors>Desaturate>Mono-mixer.

The G’mic plug-in for the GIMP (and other programs) has a more sophisticated Black and white tool, in the that allows you to specify smoothness for each channel as well as specifying the percentage. From within the GIMP: Filters>Gmic, inside of G’mic: Black and white>Black and white. The upper portion of the tool has these controls.

For information about loading these open source (free) programs: Get going with the GIMP.

Black and white effects using different mixes:

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Summary

When using a digital camera for black and white take the photo in color, not black and white mode to capture the greatest amount of information for each image. This allows you the most creative options when converting to black and white.

Because digital cameras capture images in the Red-Green-Blue (RGB) color space, the way to convert an image to black and white is to use a mix of the desaturated Red, Green and Blue channels. Every tool has some method of allowing you to specify the details of this mix. You can in some programs also isolate a particular hue to lighten or darken it for creative effects.

To be continued…

I plan to run through the black and white digital photo processing tools available in Raw Therapee and the GIMP in follow on articles.