Luminosity Mask

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Today’s Question: During your webinar on targeted adjustments in Photoshop you showed a technique that involved the Threshold command to apply an adjustment based on brightness levels in the image. Could you review the steps involved? I seem to have missed a step when I was trying to reproduce the effect.

Tim’s Quick Answer: There are various ways you can create a layer mask based on brightness levels in a photo, but one of the techniques I commonly use involves the Threshold command. The basic process involves using the Threshold command to create an image that can be used as a layer mask defined by luminance values in the photo.

More Detail: This technique is helpful for situations where you want to apply a specific adjustment to the brightest or darkest areas of a photo. Once you have such a photo opened, the first step is to create a copy of the Background image layer. To do so, simply drag the thumbnail for the Background image layer down to the “Create a New Layer” button (the blank sheet of paper icon) at the bottom of the Layers panel.

Next, change the Opacity setting for the newly created “Background Copy” layer to about 50% using the Opacity control at the top-right of the Layers panel. This won’t have any visible effect at the moment, but will be helpful in the next step.

Now choose Image > Adjustments > Threshold from the menu. This will bring up the Threshold command, which enables you to create an image that is entirely black and white with no shades of gray. The key for this purpose is that we’re able to define the tonal value at which this image will split from black to white.

Because the Opacity for the “Background Copy” layer has been set to about 50%, you are able to see the black and white image as a translucent overlay on top of the underlying image. You can then drag the slider for the Threshold command to define the appropriate split between the dark versus light areas of the photo. When you’ve found a good value, click the OK button to apply the Threshold command.

At this point you have defined the starting point for your layer mask. Bring the Opacity setting for the “Background Copy” layer back up to 100% on the Layers panel. Then choose the Magic Wand tool, and click within either the black or white area of the image depending on whether you want to adjust the shadows or the highlights in the photo.

You can now turn off the visibility for the “Background Copy” layer. With the selection active, you can now add an adjustment layer of the desired type. Be sure to increase the value for the Feather control on the Masks tab of the Properties panel to blend the adjustment into the surrounding areas of the photo.

I’ll have more tips for working with luminance-based layer masks in an upcoming webinar, so stay tuned for that!