The word “calculations” can be pretty scary for creative folk like us. It conjures up images of sweating over some really hard sums. However when we’re talking about Calculations in Photoshop what we’re referring to is the ability to blend two individual channels from one or more source images. In this tutorial, I’m going to show you how to convert a colour image into black and white using the Calculations dialog box. (And it’s not hard either!)
1. Open up your colour image that you want to convert to greyscale.
2. Choose Image > Calculations.
This will open up the Calculations dialog box. This dialog box lets you pick two channels from your colour photograph and blend them together to create a new image. (Note that you can also choose to blend two separate images together.)
3. Make sure the Preview box is checked so that you can see what you’re doing.
The default setting for the Calculations dialog box is to combine a Red Channel (Source 1) with a second copy of the Red Channel (Source 2). By playing around with the available channels and blending them together you can create a nice looking black and white image. This does come down to a certain amount of trial and error and can be different for each image you convert. The best place to start is by leaving Source 1 Channel set to Red and set Source 2 Channel to Green and see what happens.
You will notice that the default blend mode in the Calculations dialog box is Multiply. For this particular image that I’m using, Multiply works well as it gives nice dark colours to the sky. However, this is not great for every image so try changing the blending mode to some of the other modes such as Linear Burn or Overlay.
4. You can also change the Opacity setting if you feel the effect is too much. For my final image, I set up the following settings in the Calculations dialog box: Source 1 – Channel = Red, Source 2 – Channel = Green, Blending Mode = Mulitply and Opacity = 90%.
Original Image below followed by the Calculations conversion.