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All Cats Are Grey: A Beginner’s Guide To Converting Colour Digital Images to Black and White

By Association member James Reynard

Introduction

There is something about the black and white nude. Unlike painting, where a full range of subtle colours have brought centuries of nude subjects to life, the majority of art nude photographers have stripped the original colours from their nudes. Early in the history of photography there was no choice, and even when colour processes did emerge, I suspect the quality of colour available was unlikely to flatter the nude subject. However, excellent colour films, fine tuned for capturing naturalistic skin tones, have been available for decades, and in the digital age photographers have unprecedented control over hue and saturation.

Yet the nude is still at it's most powerful when rendered without its original colours. This may be down to aesthetics - without the distraction of attention-grabbing colours, the eye is drawn immediately to the line and form created by light and shade caressing the body's natural curves. It may, in part, be down to social programming; the inherent realism of photographic images helps blur distinctions between porn and art - removing the colour removes some of the realism (we do not, after, see the world in black and white, unless we spend our nights watching the commerce of cats in gardens) and thus makes the photographic nude 'more acceptable'. The latter is not a view I subscribe to, but it is a view I have heard from numerous sources over the years.

Then there is mood. Black and white, sepia, and other monotones often invoke 'the past' in a generic sense, but combined with skilled lighting they can evoke a range of more specific emotions and associations - warmth, coolness, sensuality, danger, sophisticated glamour, strength, fragility and so on.

One of the benefits (or maybe curses) of digital tools, is that there is usually more than one way to skin a cat. Adobe Photoshop facilitates the removal of colour from an image in a bewildering number of different ways. I do not intend to look at all of them, but I will explore some of the pros and cons of 6 of the main methods.

The article is aimed at those who:

Terminology

In the world of image making, the term monochrome is usually taken to be the equivalent of black-and-white or grayscale. It also refers to images containing only tones of a single colour, such as green-and-white or green-and-black. It may also refer to sepia, cyanotype, and other methods dating back to the early days of film photography.
As tempting as it is to wield the term, its ambiguous usage inclines me to exclude it altogether from the following sections. In order to avoid any ambiguity, I shall stick to the following terminology:

Black and white - images containing only black, white and grey values
Grayscale - images converted to black and white using Photoshop’s grayscale image mode
Monotone - grayscale images printed with a single, non-black ink
Duotone - following the Photoshop convention, duotone refers to monotones, tritones, and quadtones as well as duotones; duotones, tritones, and quadtones are grayscale images printed with two, three, and four colour inks

In Camera

Some digital cameras boast a black and white shooting mode. My Nikons do not, and I have never used such a function. I would be inclined to recommend against them for the simple reason that I suspect you will enjoy more control de-saturating colour images in Photoshop.

Cheap and Cheerful

To start with, it is worth reviewing, and discounting, the readymade one click methods Photoshop provides for converting an image to black and white.

Image -> adjustments -> desaturate

This method assigns equal red, green, and blue values to each pixel in an RGB image without changing any lightness values. When all three channels have the same value for a given pixel, that pixel will be black, white, or grey. This may produce a very satisfying image – or it may not. Whatever the case, you have no control over the conversion, so you cannot determine the contrast between tones in the image. Your subject may end up blending in with the background. You can try to improve matters using curves, but other conversion methods give you a far better starting point.

 

 

The hue/saturation Command (adjustment layer OR image -> adjustments menu)

Moving the saturation slider all the way to the left (-100) has the same effect as image -> adjustments -> desaturate, and therefore suffers the same drawbacks.
Since boosting colours can best be achieved in LAB mode (see my article on LAB – The Undiscovered Colour Space) I rarely use this tool, except when applying the 2-layer method described later. It is also useful when you wish to add a small amount of colour back into the image, or to selectively colour an image.
To create either effect, duplicate the colour background layer and place the duplicate above all black and white adjustment layers. Then apply the hue/saturation command and change the layer mode to colour (to partially de-saturate the image) or use layer masking to selectively remove colour.

Image -> mode -> grayscale

When using this function, Photoshop discards all colour information from the original image, maintaining the luminosity of the original. Again, you have no control over the distribution of tonal values. Also, you will probably then need to convert the image back to RGB in order to print it on an inkjet printer.
However, this mode is part of the duotone conversion process, which allows you to introduce a range of 1 to 4 colours back into images. These can be useful for creating sepia with real depth, for example. Duotones are beyond the scope of this article, but are certainly worth the time to investigate.

 

 

DIY

If you want something doing well, do it yourself. Photoshop offers various other methods for removing colour from an image that allow you to take control over the distribution of tonal values.

Image -> calculations

This command allows you to blend two individual channels into a third channel, which is black and white. When you click OK the layer will be displayed with the R,G and B channels off and the new black and white alpha channel on. You can choose which channels to blend and then choose a blending mode for the new channel; normal, multiply, lighten, overlay and soft light seem to offer the best variations. The blending opacity and masking options offer further control. If you work on the unlocked background layer, transparency is added to the list of channels in Source 1, which, combined with the red channel and a soft light blend, can produce high key skin tones. Note that this is a destructive change – you are working on the original image and the only way to revert is using history or undo – so make sure you are happy with the conversion before continuing the editing process. This method is a compromise, offering greater control than those in the previous section, but lacking the flexibility of those detailed below. It can, however, produce results quickly when time is of the essence.

The Channel Mixer (adjustment layer OR image -> adjustments menu)

Going forward, I will default to the adjustment layer approach since non-destructive editing is always the preferred approach – if you do not like the changes you have made, it is easy to revert and try again. I will also be assuming the colour space is RGB, as that is the space in which most people work. Open a new channel mixer adjustment layer (above the image layer(s)), and work on the RGB channel. Select the monochrome option. Use the source channels sliders to control the amount of detail and contrast in the image. Before making any changes, check how each source channel affects the black and white image. View the image with the red channel set to +100% and the green and blue source channels set to 0% (the default values). Repeat for the other two colour channels. Then move the sliders to create the mix you are looking for. Just as when shooting with black and white film, you may well be looking to maximise contrast between the subject and background. You will also want to ensure the main subject is the area of maximum contrast. For example, you might wish to ensure there is good tonal contrast between your model’s lips, hair, nipples and his or her skin. You may also wish to emphasise the contrast between shadow and lit areas.

The best results are achieved when the combined values of the source channels add up to 100% (or something close). Combined values of less than 100 reduce contrast and darken the whole image. Values that total more than 100 quickly blow out areas of the image. Moving the constant slider adjusts the overall luminosity of the image. Personally I would be inclined to leave this be and use the more controllable levels and curves commands.

This method offers considerable control to the photographer, but does have issues. It is not the most intuitive approach, requiring you to monitor the effects of moving 3 different sliders. You must flip your attention between the image and the channel mixer channel every time you move a slider. It can be difficult to keep track of how each change alters different parts of the image.

If you select and then deselect the monochrome option, you can modify the blend of each channel separately, to create tints. Again, this can be somewhat ‘trial and error’, with 9 sliders on 3 panels to control.

As an aside for those who enjoy their landscape photography, you can create a faux infrared appearance with the following settings:

 

 

 

 

 

 

The 2 hue/saturation Adjustment Layers Method

This is a multi-step process:

1) Create a new hue/saturation adjustment layer and set the saturation to –100.
2) Label it something like desaturate.
3) Ensure the new layer is top of the pile, and then highlight the layer beneath again.
4) Create a new hue/saturation adjustment layer but change none of the settings.
5) This second adjustment layer should sit directly underneath the first.
6) Change the blending mode to colour.
7) Label it something like tone.
8) Reopen the tone layer.
9) Adjust the tonality within the image by moving the hue slider.
10) When you feel you have the right mix, re-examine the image at 100% magnification, to ensure there is no banding or pixelation, especially in dark and transitional areas of the image.

This approach is my personal favourite since it offers the photographer great control whilst being fairly intuitive. It is also non-destructive (at least until layers are merged).

Sometimes, different parts of an image may want different hue settings. If so, create a third hue/saturation adjustment layer, set it to colour blend mode and label it tone2. Use layer masking to determine which tone adjustment layer is affecting a specific area of the image. Care is obviously required to ensure smooth transitions. The layer masks for tone1 and tone2 will probably be inverted versions of one another.
If all that seems a lot of work, especially when processing multiple images, create an action that automates steps 1-8. The remaining steps require adjustments unique to each image, so should be completed manually.

And finally…

I have my favourite method for skinning a cat - metaphorically speaking of course – in reality I am, like most cat owners, my mog’s devoted slave.
You may have a different method of black and white conversion that offers intuitive control and other benefits besides. If you do, and wish to advocate its benefits, please email me the details so I can amend this article (providing appropriate credits, of course).

It is also worth noting that later versions of Photoshop than mine (CS2) contain specific black and white conversion tools that offer highly intuitive control over the process.

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