Convert colour to black and white
Introduction
One of the benefits (or maybe curses) of digital tools is that there is usually more than one way
to do things. 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, just explore some of the pros and
cons of six of the main methods.
This article is aimed at those who:
> possess a basic knowledge of Photoshop Elements or CS
> have not explored black and white image making beyond hitting the desaturate button
> are not entirely satisfied with their current method of de-saturation
This article is not intended to be exhaustive in detail. Rather, it provides the basic information from
which you may investigate further.
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 blue and white or blue 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 discussion. In order to avoid any ambiguity, I shall use 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 Photoshop convention, monotones, tritones, and quadtones as well as duotones
Duotones, tritones, quadtones > grayscale images printed with two, three, and four colour inks
In Camera Black and White
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 much greater control de-saturating colour images in Photoshop.
Photoshop One-Click Solutions
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.
hue/saturation (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 colour) I rarely use
this tool, except when applying the 2-adjustment-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 any
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 apply 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.
More Complex Solutions
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, new, 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 mask 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.
channel mixer (adjustment layer OR image -> adjustments menu)
Going forward, I will default to the adjustment layer method 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 without losing other changes you have made. 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, 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 lighter 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 effect of moving 3 different sliders. You must flip your
attention between the image and the channel mixer panel 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.
For those who enjoy their landscape photography, you can create the basis for a faux infrared
look with the settings above.
2 hue/saturation adjustment layers
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.
In Conclusion
The best advice is to find the method that suits you best - there is no absolute right and wrong way -
but some methods offer more intuitive control than others.
It is worth noting that Photoshop CS4 contains specific black and white conversion tools that offer
highly intuitive control over the process.