Convert Color Images To Monochrome – Which Channel To Watch
Monochrome, black and white or grayscale images are sometimes far more
effective than color ones. Ansel Adams’ work is a good example, as is a
Hitchcock movie. However, since most film shot is color and all digital
cameras capture full color images, there is usually a requirement to
convert the images from color to monochrome using your favorite
image-editing program. As we shall see in this article some ways of
doing this are better than others. Part 2 will cover how to
re-introduce selected color into an image.
When professional photographers shoot monochrome film they rarely do so
without a colored filter attached. The reason for this is that film
responds differently than one might expect. Film (and the CCD in your
camera or scanner) is more and less sensitive to different parts of the
light spectrum. They are also sensitive to light outside the visible
range, such as ultra-violet and infrared. All this means that shooting
a scene on monochrome film without a filter doesn’t work satisfactorily
most of the time. The common filters we use are red or orange filters
to darken a blue sky or a green filter to improve the look of green
foliage by lightening it. Unfortunately the simplest way to convert a
digital image to monochrome is (in Photoshop) to do a mode conversion
to grayscale. Here all the color data is averaged to produce a
grayscale image. This also happens with digital cameras that have a
monochrome mode. The result is similar to using monochrome film without
a filter. It works well sometimes but we can often do better. A color
digital image is really three monochrome images (usually called
channels), one shot through a red filter, one through green and another
through blue. Often we can more easily produce a result that better
communicates what we want by using one or two of these channels only
when we convert to monochrome.
What To Do
Here is a sequence to go through when you wish to convert any type of image from color to monochrome.
1. Setting things up
Open your image in Photoshop (or any similar program like Corel
PhotoPaint), save a copy of the image with another name and then open
this copy as well. This gives you two versions of the image that you
can treat differently. You can then compare which is better.
2. Get a baseline
Convert the copy image to grayscale by doing Image -> Mode ->
Grayscale. Resize this version so that you can keep it visible on the
screen while you work on the other copy. This provides a base reference
3. Show Channels
If you don’t already have it open, bring up the channels palette using
Windows -> Show Channels. You will notice that a color image (RGB)
has four entries in the channels palette, the top one showing the
combined RGB image and three more for each of the red, green and blue
channels individually (assuming you are working in RGB).
4. Seeing red
Click on the red channel in the channel palette. This turns off all the
other channels and you see a grayscale image that is made up of only
the data from the red channel. In the red channel red objects will
appear light, blue objects dark and green objects grayish. Compare what
you get with the copy of the image which you did a straight grayscale
mode change on.
5. Green with envy
Click on the green channel in the channel palette. This turns off all
the channels except green. In the green channel green objects will
appear light, red and blue objects darkish. Compare.
6. Feeling blue
Click on the blue channel in the channel palette. This turns off all the channels except blue.
An alternate way to work is to execute a Split Channels command from
the channel palette’s menu accessed by clicking on the right arrow at
the top right of the palette. This creates three new windows called
name.blue, name.green and name.red. This can be useful with some images
where all three channels provide useful, but different, renditions of
the scene. I tend to prefer this to other approaches.
Switch backward and forwards through the channels (or the split files),
comparing them to the mode changed version. What parts of the image are
important for your intended use? Which channel gives the best rendition
of it? What parts of the image detract from your intended use? Which
channel makes it least noticeable? You should always have a clear
purpose for an image in mind. It could be as simple as showing the
beauty of a place or it could have a complex existential, political,
social or environmental message. Whatever it is, keep this in mind as
you are evaluating your options.
Remember that splitting channels is very similar to using colored
filters over the lens when shooting B/W film. A red filter is like the
red channel, etc.
If one of the channels gives a better rendition than the straight mode
conversion ensure that only the channel you want is visible and then do
an Image -> Mode -> Grayscale conversion. You will be prompted as
to whether you wish to discard other channels. Do so. Alternatively, if
you did t
he Split Channels approach earlier, just close without saving
the versions you don’t want.
Landscapes with lots of green foliage are a very common subject for
photography. Taking the green channel from a color image can give a
great result. Using the green channel adds more separation and modeling
into the foliage. This is because the well lit part of the plants will
be mostly green, thus light in the green channel, and the shadow areas
and other objects will have less green and more of the other channels,
thus being darker in the green channel. Dropping the other channels
thus increases the contrast in the foliage.
Skin blemishes, moles or acne often spoil shots of people. Unlike
landscapes where we frequently want more contrast to more clearly
separate objects, with people we usually want less contrast in the skin
tones. Since most skin has a fair amount of red in it, including
blemishes, the red channel can be a good one to use because the
blemishes will also have a substantial content in the other channels.
Examine the individual channels. In this case the red channel looks the most promising.
Convert to grayscale using just the red channel, or do split channels and use the one you want.
Apply any other manipulations required, in this case curves.
Skies can be a key aspect that makes or breaks an image. A good way to
increase the drama in a sky is to use the red channel. In the red
channel a blue sky is very dark. This makes white clouds stand out more
clearly. On the other hand atmospheric haze, one of the key ingredients
in aerial perspective (the graying or bluing out of things with
distance) is mainly in the blue channel. Thus you can increase the
sense of distance by using the blue channel.
CMYK and Other Things
So far we have worked with RGB. Yet there are reasons to work in CMYK.
CMYK offers four channels to choose from. Also the color ranges covered
by the CMYK channels can better suit some objects. For instance, many
Australian plants, such as the gum trees common throughout Australia
and also California, have a lot of blue coloration in their leaves as
well as green. For such plants the cyan channel in a CMYK version may
work better than a green one in RGB. If you can’t get the effect you
want, convert to CMYK and then try.
There is also no reason why you must use only one channel in producing
a grayscale image. For some images one only needs to remove one channel
to get the result you want. You do this by clicking on the eye next to
it in the channels palette to turn off its visibility. Some images may
also benefit from differential treatment in different parts of the
image. So you might want to use the green channel for the landscape and
the red one for the sky. A bit of masking can let you achieve this
easily. Indeed occasionally you will get an image that really benefits
from being divided up into parts and having each part treated
separately. This is really akin to the way we used to work in a
Digital cameras tend to put more noise into the blue channel than into
the others. This is because the sensor is less sensitive to the blue
end of the light spectrum and thus needs to be amplified more. This
also amplifies the noise. What this means is that even if the straight
mode convert version works well for you, it may be worth turning off
the blue channel before converting to grayscale in the interests of a