Adjust Levels of a Photo to Black and White in GIMP

There is more than one way to convert a photo to black and white in GIMP and which you choose will be a matter of convenience and personal preference. It may seem surprising to hear that different techniques produce different results, however, that is the case. With this in mind, we'll show you how you can take advantage of the Channel Mixer feature to produce more striking black and white photos in GIMP.

How to Convert a Photo to Black and White in GIMP

Before considering the Channel Mixer option, let's look at the simplest way to convert a digital photo to black and white in GIMP. Typically when a GIMP user wants to convert a digital photo to black and white, they'll go to the Colors menu and select Desaturate. While the Desaturate dialog does offer three options for how the conversion will be made, namely Lightness, Luminosity and the average of the two, in practice the difference is often very slight.

Light is made up of different colors and the proportions of the different colors will often vary from area to area within a digital photo. When you use the Desaturate tool, the different colors that make up the light are treated equally.

The Channel Mixer, however, allows you to treat the red, green and blue light differently within an image meaning that the final black and white conversion may look very different depending on which color channel was emphasized.
For many users, the results of the Desaturate tool are perfectly acceptable, but if you want to take more creative control over your digital photos, then exploring the Channel Mixer option may be best.

Adjusting Color to Black and White

The Channel Mixer dialog does seem to be hidden within the Colors menu, but once you start using it I'm sure that you will always turn to it whenever you convert a digital photo to black and white in GIMP.

First, you'll need to open a color photo that you'd like to convert to mono, so go to File > Open and navigate to your chosen image and open it.

Now you can go to Colors > Components > Mono Mixer to open the Mono Mixer dialog. Before using the Mono Mixer tool, let's just stop and take a quick look at the controls.

A screenshot of GIMP with the Mono Mixer command highlighted

The three-color channel sliders allow you to tweak the lightness and darkness of the individual colors within your photo. The Preserve Luminosity checkbox will often appear to have little or no effect, but in some cases, it can help to make the resulting black and white photo appear more true to the original subject.

Convert a Photo to Black and White with a Dark Sky

Our first example of how to convert a digital photo to black and white will show you how to produce a result with a darkened sky that will make the white of the building really stand out.

When you select Mono Mixer, you'll see that the preview thumbnail becomes black and white. We'll use this preview thumbnail to see how our adjustments are changing the appearance of our mono conversion. Remember that you can click the two magnifying glass icons to zoom in and out if you need to get a better view of an area of your photo.

Note that the colors are all set to .333. To ensure that the end results look as natural as possible, the total values of all three sliders should total 1.00. If the values end at less than 1.00, the resulting image will appear darker and a value higher than 1.00 will make it appear lighter.

Because we want a darker sky, we'll move the Blue slider to a setting of -50%. That results in a total value of -.50 meaning that the preview looks darker than it should. To compensate for that, we need to move one or both of the other two colors to the right. We can move the Green slider to .20, which lightens things like foliage of trees a little without having to much effect on the sky. We can then push the Red slider to 1.30 which gives us a total value of 100 across the three sliders.

A screenshot of GIMP with the Red, Green, and Blue sliders in the Mono Mixer window highlighted

Convert a Photo to Black and White with a Light Sky

Here we see the same digital photo in black and white but with a lighter sky. The point regarding keeping the total values of all three color sliders to 100 applies just the same as before.

Because the sky is predominantly made up of blue light, to lighten the sky, we need to lighten the blue channel. To do this move the Blue slider to 1.50. Move the Green slider to .30. And finally, reduce the Red slider to -.80.

You can see how the technique of using the Mono Mixer offers up the ability to produce different results when you convert your digital photos to black and white.

A screenshot of GIMP with the Red, Green, and Blue sliders in the Mono Mixer window highlighted
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