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Luminosity masking is a technique by which you can create layer masks from a photo based on the brightness (luminosity) values of its pixels. Such a mask can be used to add adjustments only to the bright parts of an image without touching the dark tones at all (or vice versa).
In this excerpt from my video course Mask It Like a Pro! I will show you how to use luminosity masks to get back the details in a washed-out sky. We will first create a luminosity mask. Then, we will use a Levels adjustment and the Lasso tool on the mask to refine it so that it only reveals the sky. Finally, we will add a targeted Curves adjustment layer with that mask to really pull out the details in the sky.
Here is a rough overview of the steps required to apply this technique. For detailed explanations of all the steps, watch the video above.
Step 1 – Create a luminosity mask
First, you need to create a luminosity mask that helps you isolate the sky. You do this by creating a selection from the RGB channel and repeatedly intersecting it with itself. The resulting channels will get darker and darker until the pixels that are in the dark to midtone range in the image are almost black while the highlights are still fairly bright. Chances are that you will still see some bright artifacts in the dark regions of the mask. But you can clean these up later.
Step 2 – Increasing the mask contrast
In order to create an effective mask, you can use the Levels tool Image > Adjustments > Levels to increase the contrast of the mask. Pull the White Point slider to the left until the sky is entirely white. To make the rest of the image as dark as possible (ideally it should be black), drag the Black Point slider to the left.
Step 3 – Cleaning up the mask
In most cases, there will be white artifacts left in the black parts that shall hide the remaining parts of the image. However, they should be easy to clean up using the Lasso tool (L): Draw a selection around them, make Black your background color (press ‘D’ and then ‘X’ on the keyboard), and then press the delete key.
Step 4 – Adding adjustments to the sky
Finally, make a selection from the channel that you have created in the previous steps and add a Curves adjustment layer. The selection will automatically be added as a mask to the Curves layer. The histogram of the Curves layer will be shifted to the far right of the spectrum (the tones revealed by our mask are very bright). Shape the curve such that it fits nicely around those pixels in the histogram.
Don’t make your Curves adjustment too extreme. Otherwise, the image will look unnatural quickly. Moreover, stretching out the tones in an area with little contrast too much may produce banding. Some experimentation will reveal the best setting.
Also note that putting the Curves layer inside a group and putting the mask on that group (instead of on the Curves layer), you can put any number of additional adjustment layers in that group and confine their combined effect to the sky.