21 — Selecting Sky

How about a software teaching point this month? 

One of the toughest things you sometimes encounter in landscape photography is selecting just the sky. The reason generally has to do with tree limbs, bushes, and other things projecting up into the sky and making selection tough. Worse still is when you have gaps in vegetation that lets some sky through. 

The good news is that sky is mostly blue spectrum light, so we’re going to take advantage of that. We’ll work with an image you’ve seen before on this site:

I’m not making much of a change to the image in ACR, basically just grabbing the white and black sliders and adjusting them to maximize the histogram. I don’t want you to get confused by other settings along the way. We’re just interested in that sky at the moment (and conversely, the hoodoo and foreground). I would suggest, however, that you remove any chromatic aberration, if you can, as it can impact the boundaries of the objects we’d be selecting against.

In Photoshop, go to Channels and select only the Blue channel:

You can probably already see that the lines of the hoodoo and horizon are pretty distinct, but we’re going to go further. We going to drag the Blue channel to New channel icon at the bottom to duplicate it. Your new layer will be called Blue copy, but you can rename this to something useful, like “Sky Selection” if you’d like, as I’ve done here:

Next, we’re going to do something specifically to that new channel: make sure that new channel is the only one active, then select Levels from the Adjustment menu. Bring the Black slider up until the land and hoodoo go completely black, and the White slider down until the sky goes completely white:

You probably now know why I selected this image: it’s got clouds in it that are going to be a problem, and the hoodoo itself doesn’t really go completely black. That’s okay. We’re trying to simplify selecting the sky, and sometimes we won’t get exactly what we want on one pass. I’ll quickly clean these things up by using a white brush, then a black brush:

All the above only took me a few seconds from hitting the Open Image button in ACR to getting a clean black/white mask. 

We now want the white area here to be our “selection,” so hold the Command (Mac) or Control (Windows) key and click the Channel to load its white areas as a selection. You should see the cursor change as you hold the key when you’re hovering over the Channel list, and when you click you should see the crawling ants now define your selection. (Bonus point: you might find that the Refine Edge item in the Selection menu is very helpful at disguising the actual edge. But that’s a lesson for another day.)

At this point, you’ll want to click the RGB Channel to activate it, delete the Sky Selection channel you created, and move to the Layers panel. Apply whatever layer effect you want on the sky. Invert your selection and apply whatever layer effect you want on the foreground. You’re done. 

Here I’ve done a bit of over processing on the foreground and background so that you can see that I was indeed able to get this image quickly processed based upon the quick selection I made. This is not the way I’d normally process this image, as the light cues are now screwed up. But it should be obvious that I’ve touched both the foreground and sky in different ways:

I made no attempt to refine my selection other than the bit of brush painting I did. Normally I’d take the time to look closely at every edge very carefully and be tweaking pixel level decisions. But I wanted to show you that you can make what’s otherwise a tough selection with just a simple use of the channel information. Note that here we’re working with sky, and thus the Blue channel is the thing we normally find the most useful. If you were shooting a subject with a green screen behind them, guess which channel you’d use for selection? 

Why does the hoodoo look like it’s glowing at the edge? Guess where the sun is. You’re seeing a bit of the diffractive properties of light.

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