There are so many ways to convert an digital photographs from colour to black and white it’ll make your head spin. Personally, if I know in advance I want black and white, I’ll just shoot it with film. But, sometimes, converting a digital colour image in post is your only option. The most common ways to convert images to black & white are through either Lightroom or Camera Raw or to use the Black & White adjustment layer inside Photoshop.
This tutorial from Unmesh Dinda at PiXimperfect shows another, rather novel, method of producing black and white conversions. It involves using Photoshop’s “Calculations” feature to look at the different colour channels in your image. Different blending modes are then used to produce a wide variety of options.