Science tells us there are somewhere north of 13.1 billion ways to convert an image to black and white. It's time to add another method to your arsenal, but this one is pretty cool. Here's how to use Channel Calculations to convert a color image to black and white in Photoshop.
So, what the heck are channel calculations anyway? They're pretty much what they sound like: you select a single channel from an image, do the same for another image (it could be the original image as in this video), Photoshop does some math on them (the same math it uses for blending modes), and out pops a new channel, selection, or image. Because a single channel is black and white, containing luminosity data for a single color, when you combine two channels into a single output channel, the result is again black and white, making it an interesting and efficient method for generating black and white images, as the above tutorial from PIXimperfect shows. It's a pleasingly tidy method, and the ability to cycle through blending modes and channel selections within the dialog box gives you a very quick library of resultant looks. Give it a try!