It's not unusual for photographers to want to convert a color image into a black and white one. Sometimes the image was shot on color film, or was shot in digital color, but for whatever reason, the photographer feels the image would look better in black and white. There are a number of different ways to make this conversion. This tutorial will show you how to use Photoshop's Calculations command to make a black and white image. The calculations method is great for creating contrasty black and white images that emphasize texture, particularly in skin.

First, some basics: digital images are made up of thousands of colored dots known as pixels. Scanners and digital cameras capture a grid of pixels. This grid creates a picture when viewed as a whole. The color of each pixel is determined by the different intensity (luminosity) levels of the primary colors red, green and blue. On a typical computer image, luminosity values range from 0 (none, dark) to 255 (full intensity of that color). Combining red, green and blue with this particular range of luminosity levels, a computer can create around 16.8 million different colors.

Photoshop makes "maps" of the luminosity levels of red, green and blue in each image and calls them "channels." Channels are strange things that might take a little bit of thinking to fully grasp, but it's worth understanding. For example, a reddish pixel will appear light grey or white in the red channel, because it has a high luminosity value of red. That same pixel would appear much darker in the blue and green channels. There's an additional "grey" channel which is just the overall luminosity of red, green and blue channels, exactly like the desaturated image above. You can view the different channels by clicking on "Channels" in Photoshop's Window menu, and toggling the eyeball buttons beside each one.

Now let's say we want to take this image and convert it to black and white:



The easiest way to do it is to just use Photoshop's Hue/Saturation adjustment, and drag the saturation slider all the way to the left to "desaturate" the image. Most photo editing programs have a desaturation command, so you don't actually need Photoshop to use this method. Desaturation sets red, green and blue equal values, while retaining the overall luminosity of each pixel. Making a pixel's red, green and blue values equal causes the pixel to turn grey.



The problem with the desaturation method is that the resulting image can often look flat and lifeless. The image lacks depth and texture. To me, one of the most interesting things about this particular image is the texture of the skin, and I want to emphasize it. The Calculations tool is one great way to do it.

Start with your color image, and then go to the Image menu and choose "Calculations..." You'll see the following dialog box, and your image will turn black and white instantly.



You'll probably want to play with the settings to get it right. The Calculations tool allows you to combine the red, green and blue channels of your image into a black and white image. Remember, each of the source channels actually looks like a black and white version of your photo, because it represents only the luminosity values of red, green or blue in that image's pixels.

As you can see, the Calculations dialog allows you to choose which channels to combine and how to combine them. You have a choice of blending your red, green, blue or grey channels together. It would be impossible for me to explain each and every combination and blending mode, because there are hundreds of possible combinations. However, I find the above combination of channels works well in most situations in which you wish to create a textured image. I set the blending mode to "Overlay" or "Multiply" and then decide which one I like better. I set the opacity to around 30%. Higher values of opacity cause "Source 2" to have a greater effect on the finished image. Lower opacity values cause "Source 2" to have a lesser effect.

The best way to learn what works for you is to experiment with the different settings. Don't be afraid to try different combinations of channels, different blending modes, and different opacity levels to find out what works best for your particular image.

When you've found the combination that works for your image, set the Result to "New Document" and click OK.



You're almost done. What you've got on your screen is a luminosity channel. You have to convert it to greyscale before you can continue working on it like a normal image. Go to the Image menu and select "Mode" and then "Grayscale." You can go back to Image -> Mode and choose "RGB Color" if you wish to do anything else to it that involves color, like tinting it. After you've changed the mode, you may now save your black and white conversion just like any other digital photo.

As you can see, the Calculations method is a little more complicated than straight desaturation. However, I think you'll find the results are rewarding. I hope you enjoyed this tutorial. Happy shooting!