This week, in the first of a two part series, I’m going to run through setting up basic Luminosity Masks for shadows and highlights.

Luminosity (or luminance) masking is a method for creating selections based on luminance values. Put simply, the mask is based on how light (or dark) your image is in a given area – creating a mask from the highlights will mean the lightest areas are the most effected by any adjustment, and this adjustment will taper off gradually giving an natural transition that leaves the shadow areas unchanged.

Let’s start by creating our highlight mask. Go to the Channels panel (Windows>Channels) and mouse over the top channel, our composite RGB. Hold down the Cmd (Mac) or Ctrl(PC) key and you’ll see the hand cursor now has a marquee indicating we can load a selection from the current channel. Click on the RGB channel and you’ll see a marquee, or marching ants, loaded over the highlight areas in our image. Click on the icon to save our selection to a new channel. The quick-key for loading a selection from our composite channel is Cmd+Opt+2 for Mac / Ctrl+Alt+2 for PC.

loading selection from composite channel in photoshop

Our selection is still active so I’m going to deselect and click on the new ‘Alpha 1’ channel to view it separately from the other channels.

view alpha channel separately

You should have what looks like a black and white version of your image, which essentially it is, as this is created from the luminance values in our image. But this is a mask so the light areas will be the active areas and the dark areas are will be masked off. This is our basic highlight mask so I’m going to rename the channel as ‘Highlights.

Now we need to generate a mask to give us control of the shadows and there are 2 approaches here but the net result is the same. The mask we need is the direct inverse of the highlight mask.
We can repeat the above process of loading the selection from the composite channel and then ‘Inverse’ the selection prior to saving to a new channel, or we can duplicate the ‘Highlight’ channel and then ‘Invert’. Either way, you should end up with a second mask, which we’ll call ‘shadows’, that looks like a black and white negative of your image.

invert a channel in photoshop

Let’s click on the RGB composite channel so we’re viewing in full colour again. Now we can Cmd+click (Mac) or Ctrl+click (PC) on either of the additional channels to load our Highlight or Shadow selection and make an adjustment that targets the highlight or shadow areas.

I’m going to load the ‘Highlights’ selection first and jump over to the Layers panel. Our selection should be active so any adjustment layer I now create will use this selection to create a layer mask. I’m going to add a Solid Colour layer so we can see where our mask is working.

add solid colour layer in photoshop

We can see our colour being added in the highlights but tapering off as the mask darkens leaving the shadow areas unaffected. Now I’ll change the layer Blend Mode to ‘Overlay’ so the colour fill adds a warming effect to our highlights.

color fill with overlay blend mode

Now let’s repeats the process with our ‘Shadows’ mask and add another colour fill layer.

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Again we can see how the adjustment is contained to the shadow areas and tapers off as it reaches the highlights. Lets change the Blend Mode to ‘Overlay’ as before.

Below we can see the original colour on the left, with the adjustment on the right. In this example I’ve used the masks to take the shadows and highlights towards different Hues and the effect is quite subtle as I’ve chosen muted colours.

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Luminosity masks can be used for tonal and colour adjustments to a single image or used in compositing – photographers will often use them to combine different exposures of the same scene where the shadows and highlights can’t be captured in a single exposure.

In part 2 of the series we’ll look at creating a ‘Midtone’ mask and how to create additional highlight and shadow masks that target several different levels of luminosity independently giving multi-level control over your images.

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