Luminosity Masks: What They Are and How to Use Them

Post-processing can make or break an image. It doesn’t matter how much you change in your photo editor of choice, even a small adjustment can damage your image if it isn’t applied correctly.

A common mistake I see with post-processing is applying all adjustments globally (i.e. to the entire image). It’s something we rarely want, which is why we tend to use Lightroom’s Highlights and Shadows sliders to adjust the exposure rather than the Exposure slider. And once you bring your image into Photoshop you can apply more advanced techniques and adjustments.

More than ever, you need to know how to make these adjustments correctly.

And that’s where luminosity masks come into the picture.

What are Luminosity Masks?

If you’ve read any of my previous articles you may have seen me talk about selective post-processing – making adjustments that only affect specific areas rather than the entire image.

Luminosity masks are selections based on a pixel’s luminosity value. This means you can accurately select only the bright, dark or midtone pixels. We can refine these selections to affect only the brightest brights or the darkest darks, and use them as layer masks for our adjustments.

Since they’re based on the pixel’s brightness, we can get extremely accurate selections that target only the specific pixels we want. Having an accurate selection means we avoid certain unwanted artifacts you might otherwise experience.

You won’t find luminosity masks in a list or menu within Photoshop (although third-party plugins can automate the process). Instead, you need to create them manually by making selections based on the RGB channels.

How to Use Luminosity Masks

Now that you know what they are, the next thing you need to know is how to use them. As you probably know they don’t adjust the image themselves. Instead they’re a selection you can apply to any layer or group you can use a layer mask on.

Before we look at how to use them, we need to look at how to create them. You can do this either manually or by using a third-party plugin. I strongly recommend you learn how to create them manually before you start using a plugin to speed up your workflow.

How to Make Luminosity Masks

Let’s create a Brights mask, which will select the bright areas of the image but leave the midtones and darks untouched. Keep in mind this is the broadest brights mask, and you’ll probably need to refine it to target more specific pixels. (More on that another time.)

Start by opening an image in Adobe Photoshop, and follow these steps to create the mask:

1. Hold down Ctrl/Cmd and click the RGB thumbnail in the Channels Tab. You should now see marching ants around several areas of your image.
2. Save the selection by clicking the Save selection as channel icon. The selection is saved as a channel and given the name Alpha 1.
3. Double-click the name of your new channel ‘Alpha 1’ and rename it to ‘Brights 1’.
4. Hold down Ctrl/Cmd and press D to deselect the selection.

Not too hard, right?

This is what the Brights 1 Mask looks like

We’ll make the Darks mask next. It’s pretty much the same process as making the Brights mask except we need to invert the selection:

1. Hold down Ctrl/Cmd and click the RGB thumbnail in the Channel Tab.
2. Hold down Ctrl/Cmd and Shift, and press I to invert the selection.
3. Save the selection.
4. Double-click the new channel’s name and rename it to ‘Darks 1’.
5. Hold down Ctrl/Cmd and press D to deselect the selection.

This is what the Darks 1 mask looks like

Finally, we’ll create the Midtones mask. This one is made slightly differently to the first two masks.

1. Select the entire image (hold down Ctrl/Cmd and press A).
2. Subtract Brights 1 (hold down Ctrl/Cmd and option/alt, and click on the ‘Brights 1’ channel’s thumbnail).
3. Subtract Darks 1 (hold down Ctrl/Cmd and option/alt, and click on the ‘Darks 1’ channel’s thumbnail).
4. Save the selection and rename the new channel to ‘Midtones 1’.

This is what the Midtones 1 mask looks like

We’ve now created the three basic luminosity masks. The process might seem confusing at first, but soon you’ll find creating masks as easy as one, two, three.

How to Apply and Use a Luminosity Mask

Now that we have our masks, let’s look at how to use them. As I mentioned earlier, you can apply luminosity masks to any layer or group you can use a layer mask on. This includes merged layers, adjustment layers, groups, smart objects and more.

I want to brighten the darkest parts of this image but leave the highlights alone

A typical processing scenario is the foreground being is a bit too dark while the sky is perfectly exposed.We can fix this by increasing the exposure using a Curves Adjustment. But using a Curves Adjustment without a mask will brighten not only the shadows,but also the areas that are already well exposed.

So let’s use the Darks mask.

Hold down Ctrl/Cmd and click on the Darks channel’s thumbnail to activate the selection. (You’ll know it’s active when you see the marching ants.)

With the selection active, create a Curves adjustment layer. Since the selection is active, the Darks luminosity mask will be applied to the Curves’ layer mask. Any adjustments you make on this particular layer will only affect the areas represented by white on the mask.

Now simply pull the Curve up to brighten the darks. You can toggle the mask on and off by shift-clicking the layer mask to see the adjustment with and without the mask. (It makes a huge difference.)

With the luminosity mask applied

Without the luminosity mask applied

What Now?

This is just one way you can use luminosity masks. When processing an image I use them several times with a variety of adjustments. They can even be used to blend multiple images.

And while third-party plugins can automate the process for you, you really should learn how to create them manually first. Understanding how they work makes it easier to know how and when to use them – and when not to.

If you’re interested in this subject, take a look at my eBook A Photographer’s Guide to Luminosity Masks where I teach you everything you need to know about them, as well as a variety of other masks and advanced selections.