Photo Realistic Drop Shadows in Photoshop

Step 3: Copy The Background Layer And Paste As A New Channel

In the Layers palette, click on the Background layer once again to select it:

With the Background layer selected, press Ctrl+A (Win) / Command+A (Mac) to select the contents of the layer, which in this case is the original water photo. You'll see a selection box appear around the image in the Document window. Then press Ctrl+C (Win) / Command+C (Mac) to copy the layer contents.

Now switch to your Channels palette, which is grouped in right beside the Layers palette, and click on the Create New Channel icon on the bottom of the Channels palette, as circled in red below:

A new channel filled with black, which Photoshop names "Alpha 1", will appear below the existing Red, Green and Blue channels:

With the "Alpha 1" channel selected, press Ctrl+V (Win) / Command+V (Mac) to paste the contents of the Background layer into this new channel. You'll see the image appear in black and white inside the thumbnail preview area of the "Alpha 1" channel:

And you'll see the same black and white version appear in your Document window:

Press Ctrl+D (Win) / Command+D (Mac) to deselect the image at this point. We're going to apply a Gaussian Blur filter to the new channel next.