Black and white conversion – A how-to guide

You’re shooting in colour, and now you want to convert it into black and white. Here’s a tutorial / informative article on how you should convert your photos using photoshop (or any other editors such as Gimp). I got the inspiration to write this article when I stumbled upon a similar writeup on black and white conversion in a recent issue of Landscape Photography magazine.

Note: Grey/Gray are used interchangeably in the article. It’s simply a matter of convention. Grayscale is used in Photoshop and other US-based editors.

We all shoot in colour, most of the time.

Majority of us who uses digital cameras shoot in colour – the sensors in our camera collect data from each colour channel (red, green and blue) and the processing unit in the cameras will then intrapolate data among neighbouring sensor units to yield the final photo, which is compressed and saved in JPEG format. The RAW format simply means that the file (usually 2-4 times larger than the original JPEG file at the same resolution) contains all the data collected by the sensors that are yet to be interpreted by the algorithm built into the camera’s processing unit.

Some photographers may want to convert their colour photos into black and white during the post-processing step. Usually it’s safer to shoot in colour and then convert the image into black and white because vice versa is impossible due to the exclusion of colour data in black and white photography. Now you’re stumped – what should you do to convert a colour photo into black and white? You’re presented with two choices – (1) desaturating the photo or (2) converting the image from the RGB mode into grayscale. Both methods will yield monochromatic, black and white photos but they may give results that differ – whether the difference is barely noticeable or great depends on the colours in the photo.

To desaturate an image in Photoshop, simply go to Image > Adjustments > Desaturate or you can use the shortcut [Shift] + [Ctrl] + [U].

To convert an image to grayscale, go to Image > Mode > Grayscale. The RGB option should be selected by default and a warning dialogue box will popup when you select the grayscale option. Proceed with the conversion anyway. Alternatively, you may convert the image into LAB colours and then discard all other channels (channels a and b)except for the lightness channel.

Note: You can control how the conversion to grayscale is done by applying a Black & White adjustment layer in Photoshop. This feature is only found in versions CS3 and above. For those using CS2 and below, you can use channel mixer to achieve the desired results when converting an image to grayscale.

The colour palette

Here’s how a standard colour palette (the default one as seen in Photoshop) will appear when we apply desaturation (center) and grayscale (right) to the palette.

The difference between a desaturated and a grayscaled palette becomes a little more obvious here. If you do look at the top row of the palette, the colours red, yellow, green, cyan, blue and magenta all share the same shade of gray upon desaturation. However, the grayscaled version showed them in different shades. Why is that so? The answer is simple – desaturation merely removes colour information (well, that’s how it should work anyway) while grayscaling does the same thing but takes into account the luminance value of the colour. That’s why grayscaling an image is known as luma-preserving the image as the luminance is preserved as it is.

Different colours have different luminance values – for example, green has a higher luminance value than blue because our eyes are in fact, more sensitive to the former than to the latter. Therefore, grayscaling an image actually takes into account the brightness perceived by the human eye while desaturation doesn’t. By the same principle, we can argue that grayscale approximates the contrast of the scene we would see – if we see black and white only – better than what can be achieved by desaturation. In general, desaturated images will have less contrast than greyscaled images. However, it should be noted that the loss of contrast will not be as drastic as depicted in the colour test strip above because in reality, photos have hues of varying degrees of saturation and therefore the difference in colour intensity (not luminousity) will still be able to lend sufficient contrast to a desaturated image.

Photo samples: desaturating vs. grayscaling them

In this section, we will be comparing the different results obtained by desaturating or by greyscaling an image. I’m using a few sample photos here – the colour photo (left) followed by a desaturated version (center) and a grayscaled version (right). The photos may be viewed in a larger resolution if you choose to – very useful when you want to scrutinize the slight differences between the two monochromatic versions.

Sample 1: Sky scraper and the sky

Skyscraper and the sky: The sky is a slightly darker in the greyscaled image, and more importantly, the building’s beige and brown facade enjoyed better contrast in the grayscaled image as compared to the desaturated image.

Sample 2: Don’t jump

Don’t jump: The area of photo where the toes and slippers are showed better contrast. Differences can also be detected between the bright and shaded regions of the lawn on the left.

Sample 3: Day out at Sentosa Beach

Day out at Sentosa Beach: The shade of blue on the ball is better contrasted against the yellow in the grayscaled image.

Sample 4: Food hunting in town

Food hunting in town: Note the difference in contrast at the area of the photo where the cup with straws above it – in the grayscaled image, the inner surfaces of the straws are slightly darker, and more details can be seen on the cup as well.

Sample 5: A florist’s little world

A florist’s little world: The grayscaled version showed considerable better contrast than the desaturated version, especially around the area between the cupboard and the pillar.

Sample 6: Mommy and gummy bears

Mommy and gummy bears: This is one of the better examples for comparison – check out the gummy bears with warmer colour tones. In the desaturated version, there is little contrast between the red, orange and yellow gummy bears while contrast is better in the grayscaled image.

Verdict

Grayscaled photos showed better contrast than desaturated photos, and this effect is well observed in photos with warmer tones or with objects of warm colours. The degree of contrast differences between photo varies depending on the scene, lighting and colour temperature. Warmer coloured photos will most probably show more contrast than cooler photos when you compare their desaturated / grayscaled versions side by side. Therefore, I recommend converting an image to greyscale instead of desaturating it for the effect.

Even better: Channel mixing

Sometimes, grayscale conversion does not best preserve all the tones in an image – if you would want to selectively brighten/darken certain colours in a black and white image, channel mixing is the way to go. Let’s say we want to convert the following photo into black and white, using channel mixing.

download: Black & White conversion test photoVersion 1.0 \ File size: 1.48 MB \ 63 downloadsA test photo with a resolution of 1800*1200 pixels for the black & white conversion tutorial using the channel mixing method.

You can download the sample image above if you would want to follow the channel mixing tutorial step-by-step. It’s not absolutely necessary though, especially if you’re well-versed with Photoshop and have your own photo to start off with. In this short tutorial – I promise, will take you less than 15 minutes – I will be using Photoshop CS4. The adjustment layer feature is only available in Photoshop CS3 and above. However, worry not if you’re using version CS2 and below. I’ll get that covered as well.

1. Create new channel mixer adjustment layer

Open up the image in Photoshop. Check the bottom bar of the Layers panel and there should be an icon that looks like this: . When you hover over the icon (which looks like a yin-yang symbol with a tiny inverted triangle at the bottom right) it should say “Create new fill or adjustment layer”.

Click on it and a menu should appear. Select Channel Mixer and a new channel mixer layer should appear. And yep, it is from the same menu where you can select the Black and White adjustment layer as well.

For users on CS2 and below, it gets a wee bit more complicated. You will have to duplicate the original image (by the way, it’s standard practise) by hitting [Ctrl] + [J]. Hide the original layer and then go to Image > Adjustments > Channel Mixer. Unlike CS3 and above, you cannot modify the channel mixer settings once you have applied them, so trial and error will be a little more troublesome with CS2 and below. The History panel or the undo command will be helpful in this case.

2. Modify the channel mixer adjustment layer

This is where you let your creativity roam free, guided only by experience, inspiration and intuition.  An adjustments menu should appear when you create the new adjustment layer. If it doesn’t, simply click on the black and white circle of the adjustment layer in the Layers panel.

You may select a range of preset channel mixer settings, or you can customize your own mixer. For starters, you can pick one of the presets and modify them from there onwards to your liking. Once you familiarize with how channel mixer works, you can directly customize your mixer.

To customize the channel mixer for black and white conversion, check the Monochrome option. The output channel should change from Red/Green/Blue (usually it’s Red by default) to Gray. Your photo will change into a monochromatic image instantly. However, that’s not the end of the story yet – see the red, green and blue sliders? By default they’re set to 40%, 40% and 20% for the RGB channels respectively.

For the sample photo, we want more contrast between the building and the sky, so I would want to drag the blue channel down so that blue is ‘under-represented’ and therefore the sky, which is predominantly blue, will appear darker. The exact settings depends on your personal tastes and preferences, and the following settings are just my own – for your reference only. You would notice that once you start dragging the blue slider leftwards, the total percentage (100% by default) will start to fall. No worries, because we’ll be tweaking other channels later.

Once you have achieved the desired contrast for the blue sky, now drag the red slider rightwards. Since the building’s facadé is predominantly red and I want to brighten it, I will increase red channel’s share. As you drag the red slider rightwards, notice that the total percentage will rise again.

Done with the red slider? Now tweak the green slider until the total adds up to 100% to maintain the correct exposure as the original image. You do not need to stick to 100% though – then again, it depends on you. You can also play with the contrast slider, too.

For users on CS2 and below, the same applies to the dialogue box that pops up when you select Channel Mixer from the overhead menu.

3. Final result

Here is the final result of the greyscaled image customized using channel mixer:

Finally, here’s a useful comparison between the different methods of black and white conversion techniques we have previously discussed:

The differences between the desaturated and grayscaled versions have been covered earlier on. For the channel mixer version, we have selectively darkened the blue sky and brightened the facadé and the contrast is better than the grayscaled version.

4. Verdict

While grayscale conversion is generally preferred over desaturating an image, if you have more time on hand and are willing to invest some time in more detailed, customized post-processing, channel mixing is definitely the way to go.

References and resources

So, it’s a wrap! Here are some other tutorials and articles that I have referred to when penning this tutorial and you may find them useful as well.

1. Grayscale vs. Desaturate for Black and White printing
2. Desaturate vs. Graysale Mode
3. Definitive Guide to B&W Conversion