Put color back in the sky with luminosity masks

May 26, 2014

Do you ever look at your photos and think “the sky was much more colorful than that!!” You can use brushes or gradients in Lightroom or Photoshop, but things can get tricky around buildings, mountains or other parts of the skyline. Luminosity masks offer a powerful way to let you specifically select the parts of the sky you want to adjust, without bleeding over into areas you want to leave alone.

Here’s a typical image you might take after sunset. This is the beautiful Sunset Cliffs area of San Diego, California. I did as much as I could in Lightroom (boosted overall vibrance, added a light gradient mask, added clarity, and boosted overall contrast), but the sky still lacks some of the vibrant color I saw. So I felt it was a good time to switch to Photoshop and finalize the image with a simple luminosity mask.

STEP 1: Generate the Luminosity Masks

Luminosity masks are simple layer masks based on the brightness of the image. You can create masks that preferentially select highlights, shadows, midtones, and numerous variations in between to get more specific. The beauty of luminosity masks is that they are built from the image, which means helps make it easy to create seamless transitions from the areas you are affecting to the areas you don’t. The masks can get a little tedious to make for every image, so I’ve creates some free luminosity masking actions you can use on your images. Just double click the file and Photoshop should install the action. Then run the “Create All Luminosity Masks” action. This will create 19 channel masks, which you can see by clicking to the channels tab. Each of these channels represents a different luminosity selection. Lights represent the highlights (with 1 being the most broad, and 5 being the most selective to just the brightest highlights). Darks do the opposite to let you select shadow areas. The midtones represent the tones not selected by the Light/Dark mask of the same number (so 1 is the least selected, and 5 is the most expansive- including everything but the darkest shadows and brightest highlights). There are also some offset midtones masks if needed.

STEP 2: Select the right mask

Because we’re adjusting a bright sky, we want to use a Lights mask. By clicking on the various channels, you can see what portions of the image will be selected in (more black) or masked out (more white). Note that in the channels tab, these masks appear the opposite of how they will in the layers panel, so it can be a little confusing. In this case, I think the Lights (2) mask shows a pretty nice selection of the sky, while at the same time not selecting the palm trees and other foreground elements we want to mask out. To load that channel as a selection, simply <command>-click the channel icon (on a PC, use the <control> key). You’ll see some “marching ants” that let you know the selection has been loaded. Now click on the colored RGB channel at top (don’t use any modifier keys) to go back to viewing your image.

STEP 3: Create an adjustment layer

Click back to the layers tab, and choose the option at the bottom to create a new layer adjustment (or go to the menu under Layer / New Layer Adjustment) and select “curves”. This will create a new layer and you should see your mask has been applied to it. This mask will restrict the adjustments you make to just the selected pixels (proportional to how white the mask is for a given pixel – remember “black conceals and white reveals” for layer masks). You should then adjust the curve to darken the sky until it looks nice. You’ll see in my curve that I made an S shape. This helps keep contrast in the sky, as I don’t have any true white. A good indicator of this is the histogram in the curve, which shows that all the tones I’m adjusting are in a pretty narrow band of very bright midtones, but not true highlights. Put another way, you should drag down the curve around the beginning of the histogram data, and pull it up around the end.

STEP 4: Mask the luminosity mask to apply it selectively

Now our sky looks great, but the buildings and cliffs in the foreground have also been darkened, and that adjustment doesn’t look very nice. So, we need to restrict the area where the luminosity mask is applied. We could edit the luminosity mask itself, but if we make a mistake, it becomes very hard to undo. So the best approach is to apply a mask to the mask! With the adjustment layer selected, just hit <command>-G (PC: <control>-G) to put it into a group. Next, click the “add layer mask” icon at the bottom (or on the menu: Layer / Layer Mask / Reveal all). Now just paint black across the bottom of the image to conceal that part of the adjustment. While luminosity masks are pretty selective, you’ll want to zoom in and be somewhat careful around the transition areas. But as you can see from my mask below (which you can show by <alt> or <option> clicking the mask), I didn’t have to get all that specific. If I didn’t have a luminosity mask, I’d have to create an incredibly detailed mask using the full suite of Photoshop’s selection tools, which would have taken much longer.

We’re now done. Here’s a zoomed section showing the original image.

And here’s what the image looks like with our mask. Notice that the result looks completely natural – there are no white spots in the sky, and the trees haven’t been darkened.