So why do we need so many ways to convert a color image to black and white? Light users of Photoshop and non-photographers are not aware of this, but photographers, especially those who are seriously into black and white, know the one problem photographers faced even before digital technology: that some colors reproduce as almost the same tones (shades of gray) in black and white (B&W).
Take for example an image, which is a picture of a red tomato surrounded by green vegetables. When converted to 88W, both the red and the green colors have the same gray tones, diminishing the distinctiveness of the red tomato.
What photographers did when shooting with Black & White film was to use color filters over the lens, called contrast filters. Through a red filter, the red tomato turned out much lighter than the green vegetables. Through a green filter, the red tomato appeared darker while the green leaves appeared much lighter. The desired effect was easy to remember: a filter lightened its color and darkened its opposite color. Thus, we used a yellow or red filter to darken blue skies, a blue one to lighten them.
In digital photography, we can use Photoshop's 'Hue/Saturation' or 'Channel Mixer' commands to achieve the same result as contrast filters. They allow us to manipulate the colors in our RGB images to give us better control over the tones during conversion to B&W. Photoshop will sample the color and if it is a combination of the basic colors, will list down that color in the drop-down menu.
Hue & Saturation
Step 1: Open the file
Open the file to be converted into B&W. It may help if you do some tonal adjustments before converting the image to B&W.
Step 2: Duplicate the Image
It would also help to simultaneously view a color version of the image as you manipulate the colors in your image during conversion to B&W. To duplicates go to Image > Duplicate.
If necessary, reduce the windows so you can see both documents at the same time.
Step 3: Choose Hue/Saturation
At the bottom of the 'Layers' palette, choose the 'Create new adjustment layer' option and 'Hue/Saturation Adjustment Layer' from the drop-down menu.
Note: Choosing an adjustment layer rather than choosing the adjustment command (under the 'image' menu) gives you more flexibility.
Step 4: Desaturate the Image
In the Hue & Saturation dialog box, desaturate the image by moving the Saturation slider all the way to the left (-100). This removes all the colors in your image, making it monochromatic. I
Step 5: Adjust the Tones
Although the image is now monochromatic, you can still lighten or darken the Individual colors that make up the image, increasing their tonal separation.
Channel Mixer
Step 1: Open the file
Open the file to be converted to B&W. Sometimes it helps if you first adjust the contrast and brightness of the Image before conversion.
Step 2: Duplicate the Image
As In the first technique, it will help when manipulating the colors in your image during conversion if you have a color copy of the image for reference. To duplicate, go to image > Duplicate.
If necessary, reduce the window so you can see both documents at the same time.
Step 3: Choose Channel Mixer
At the bottom of the layers palette, click on the Create new adjustment layer icon and choose Channel Mixer Adjustment Layer.
Step 4: Desaturate the Image
In the Channel Mixers dialog box, click the Monochrome option to turn the color image into grayscale.
In this example, when we converted the Image into grayscale the red orchid lost its prominence.
Step 5: Adjust the Tones.
Although the image is now monochrome, you can still lighten or darken the individual colors that make up the Image, increasing their tonal separation. Unlike in Hue/Saturation above, in Channel Mixer, you are presented only with Red, Green, and Blue sliders. Moving the Red slider to the right (increasing the percentage) tightens anything that has a red component in the Image (remember the rule of thumb of contrast filters). It will also lighten the overall image, as you are effectively increasing the intensity of light! To avoid overexposure, you have to keep the total of the three sliders within 100%.
