How to Create and Use Luminosity Masks in Adobe Photoshop

What You’ll Be Creating

Creating good, dependable, seamless selections in Adobe Photoshop can be excruciatingly slow and difficult. However, the program has entire toolsets devoted to making this task better, faster, and easier. 

1. How to Create the Selections

The selection techniques here will work on any image, but to follow along with the retouching steps, you will need to download this image:

• Silverback Gorilla

Step 1

The first selection is of the luminous values of the image. Creating this selection is fundamental to this technique as the other selections are all based on this one. 

As important as this is, there’s no menu or tool to be found to create it. It’s almost like Photoshop’s secret handshake. The keyboard shortcut is the hand cramping combination of Alt-Control-2. Prior to CS5, it was Alt-Control- ~ (yes, that’s a tilde!) But the easier method to create the selection is to go to the Channels panel and Control-click on the composite RGB channel. This creates a selection around all the luminous or highlight areas of the image.

Step 2

Click on the Save Selection as Channel icon at the foot of the Channels panel. This creates a new channel automatically called Alpha 1. Rename the channel to Highlights.

Step 3

Next, intersect this selection with itself. Do that by holding down Control-Alt-Shift while clicking on the Highlights thumbnail. This selects a brighter subset of the highlights selection. Save this as a new channel called Bright Highlights.

Step 4

Intersect the Bright Highlights selection with itself (again, that’s by holding down Control-Alt-Shift while clicking on the thumbnail) and save this selection as the Brightest Highlights.

That now provides selections for three different levels of highlights to work with. You could keep going by continually intersecting the selection with itself, but I’ve never found the need to go further than these three. It’s time to turn our attention to the shadow areas.

Step 5

Reselect the Highlights selection by Control-clicking on that channel. Then go to Select > Inverse (Shift-Control-I)—this changes the selection to be the opposite of what it was. That means instead of it being a highlights selection, it’s a shadow selection. Save this as a new channel and name it Shadows.

Step 6

Use the same self-intersection technique to create two more channels of increasingly darker selections named Darker Shadows and Darkest Shadows, respectively.

Step 7

So with three highlight channels and three shadow channels, what’s left? The midtones, of course! Start by selecting the entire canvas with Select > All (Control-A), and then subtract the highlights from it by holding Control-Alt while clicking on the Highlights channel. Then subtract the Shadows from the resulting selection in the same manner.

At this point, Photoshop might give a warning about the selection edge not being visible because no pixels are more than 50% selected. This just means that a selection will be active that will not have the “marching ants” around it.

Save this new selection as Midtones. There’s no need to intersect this channel with itself as that produces an empty selection.

Always finish up by returning to the composite RGB channel before resuming work in the Layers panel. This series of steps works on every image, regardless of what it is. Consider recording an Action to generate these channels at the press of a button, especially if you tend to do a lot of photo editing and retouching.

2. How to Use the Selections

The luminosity selections are now safely stored away in the Channels panel. So how are they any benefit? Why did we even bother to create them? Because they allow targeted adjustments that would be difficult, if not nearly impossible, by any other selection means.

Step 1

Control-click the Highlights channel to create a selection. Then, back in the Layers panel, add a Curves adjustment layer (Layer > New Adjustment Layer > Curves). Photoshop automatically uses the selection as a mask on the adjustment layer. So pulling the midpoint of the curve upwards will only brighten the highlights in the image and leave the midtones and shadows alone.

Step 2

Load the Shadows selection by Control-clicking on that channel and add another Curves adjustment layer. This time, pull the midpoint of the curve slightly downwards to deepen the shadows.

Step 3

The midtones selection can be used to create beautiful and subtle lighting effects without worrying about affecting the highlights and shadows. Load the Midtones selection and add a Hue/Saturation adjustment layer. Click the Colorize option and set the sliders to a color cast of your choice. The values I used are below.

• Hue: 33
• Saturation: 46
• Lightness: +8

Just for the purpose of illustration, try Shift-clicking on the mask thumbnail to temporarily turn off the mask. See how strong that colorize effect is without the mask? Yuck! Imagine trying to paint out all those highlights and shadow areas by hand! That alone shows just how useful this technique is. Just be sure to Shift-click the mask again to reveal it.

Step 4

Add a New Layer (Control-Shift-N) to the top of the layer stack and name it Highlights Dodge. Then go to Edit > Fill and select 50% Gray for the Contents and hit OK. Set this layer’s Blending Mode to Overlay to render all the gray invisible. Use the Dodge Tool with the Range set to Midtones and Exposure at 9%. Softly begin brushing over the highlight areas to further expand them.

Step 5

Load the Highlights selection and use it as a Layer Mask to restrict the dodging to only the highlight areas. Then look in the Properties panel and reduce the mask’s Density to 60% to fade that restriction.

Step 6

Use the same technique to create a Shadows Burn layer. Only this time, use the Burn tool to deepen the shadow areas and use the Shadow selection for the layer mask.

Step 7

Create a merged layer at the top of the layer stack by holding down the Alt key while going to Layer > Merge Visible. Then go to Filter > Sharpen > Smart Sharpen and adjust the settings to be slightly too sharp. Here are the settings I used for this image:

• Amount: 207%
• Radius: 0.9 px
• Reduce Noise: 7%

Step 8

Load the Brighter Highlights selection and use it as a Layer Mask on the merged/sharpened layer. This restricts the sharpening effect to only the brighter highlight pixels. Reduce this restriction slightly by adjusting the mask Density to 81%.