If there’s one dialog in Photoshop that scares the living daylights out of people, it’s the Calculations dialog. It’s got an awfully intimidating layout for a dialog that simply lets you combine channels, and that’s part of the beauty of it—once you learn this technique, you can “name drop” with it to impress other Photoshop users. For example, if you’re talking color-to-grayscale conversions, just mention in passing, “Oh, I don’t use Channel Mixer. I do my conversions using Calculations,” and they’ll act like P. Diddy just walked in the room. Bling, bling!
Open the color photo that you want to convert to grayscale ...