Use the calculations command to mask from channels - part 1

I'm not going to take you through how to mask with channels in great depth as there is already another tutorial covering this. What I will be doing is demonstrating how to use the calculations command to create a mask with less work brushing out a chosen channel.

First you will need to take a copy of the blue channel (or which ever has the strongest shadows and outlines) by dragging the channel over the "create new channel" icon second from right in the channels window.

Next invert the channel using ctrl+I.

Next we will adjust the levels of the new channel by opening the levels window (ctrl+L) and adjusting the amount of white and black in the channel. I used the settings shown in the example window for this image.

Next we will need to duplicate the channel with the best highlights, which for this image is the red channel, and adjust it's levels also.

Now we have our two duplicated and adjusted channels we can call upon the "calculations" command ( found under image, calculations). Open the calculations window (shown in sample image) and make sure you have the current image selected for both source 1 and source 2 and that background is the layer you have selected too.

Next we need to change source 1's channel to the red copy (renamed highlights in my example) and source 2's channel to the blue copy (shadows) making sure that the invert selection boxes are unchecked.

before accepting the calculations command we need to choose the best blending option for the image which for this image was "hard mix" or "vivid light" (either of these is usually a good bet).