We’ve all seen them… awful, unnatural looking transitions between foreground and sky or subject and background. Light “halos” at the edges which serve as indisputable evidence that you’ve Photoshopped the image. In most cases, quick selections resulting in hard-edged masks are the culprit. Feathering helps, but only so much.
I’ve gotten great results working with a technique called “channel masking” (a.k.a. luminosity masking). Using this technique, I can make natural looking adjustments without even touching a selection tool. This blog is a quick introduction to the technique (and nothing more) and assumes you have at least intermediate skills with Photoshop. The applications of channel masking are pretty extensive. With that in mind, let’s darken a sky.
We’ll start with the following image of Deadvlei in Namibia. I know this blog is not about Deadvlei, but I have to say it… Deadvlei is super cool.
For this image, I’m happy with the sky as it is, but let’s pretend I’m not. Let’s darken it. With the image open in Photoshop, click on the channels tab as seen in the lower right-hand corner. You’ll see the RGB channel as well as the individual red, green and blue channels.
You’ll notice I’ve clicked on the blue channel which is now active. The blue areas of the image are lighter in tone, indicating that blue is stronger in the blue channel (imagine that!). The color blue is weaker in the darker areas of the channel. If the red channel was selected, the dunes will appear lighter. Let’s stay with the blue channel because we are darkening the blue sky.
You will recall that, with masks, the lighter areas are more affected than darker areas when making an adjustment. We are going to turn this amazingly detailed channel into a mask that you could never create using any brush or selection tool in Photoshop. To do that, hold down Command (Mac) or Control (Windows) and click on the blue channel layer. The “marching ants” appear around the 50% brightest pixels in the image, as in the following screenshot:
Now, in this example, I will just keep the 50% brightest pixels selected for my adjustment. However, if you hold Command-Alt-Shift (Mac) or Control-Alt-Shift (Windows) and click again on the blue channel, you will cut the % in half each time you click (1st click means 25% brightest pixels, next click 12.5%, etc). The more times you click, the more your adjustment will be limited to the lightest pixels in the image as they appear in the blue channel.
After your selection is created, click on the layers pallet and create a curves adjustment layer. (We won’t actually be modifying the curve here). Change the blending mode to Multiply to darken the image by one stop. The light areas of the mask are darkened most in the image and dark areas the least. Below is the result, which you can compare to the first image in the blog. Notice the sky is darkened, but the rest of the image is not as affected. No ugly halos. Perfect transitions. You can now adjust the opacity to control the amount of darkening.
For more control, you can create a new folder with a mask and drop the adjustment layer in that folder. Then, paint on the folder mask to limit the adjustment to the lighter areas of the folder mask. With this image, it would be easy to paint gray or black over the ground to limit or eliminate any adjustment to that part of the image. Also, keep in mind that darkening the selection is not the only option here. You can lighten, add saturation or make other adjustments.
Again, that is just a very brief introduction to the technique. If you want more info on channel masking, you’ll want to learn from a real master. I’d recommend checking out Sean Bagshaw’s video series “Extending Dynamic Range” or “The Complete Guide to Luminosity Masks”.