Image Blending With Luminance Masks

In a recent article we looked at using luminance and density masks as a tool for sharpening images.  Today we’re going to look at using luminance masks as a tool for blending images to extend dynamic range.  It’s a very slick trick and much easier than manual blending using layer masks and painting in the image.


Here we’ve got two images of a lighthouse shot at sunset.  This is a difficult setting, particularly shooting into the sunset.  If you set the exposure for the sunset, the foreground is rendered in silhouette.  If you set the exposure for the lighthouse and foreground, the sky is overexposed.  In a previous article on Blended Exposures I showed a couple ways these two images could be blended manually.  This technique of using luminance masks will automate that process.

After opening the files into Photoshop, copy the one with the overexposed sky and paste it as a new layer onto the image with the underexposed foreground (you can do the opposite and your mask selection will be the opposite).  With the top layer active (the overexposed sky), press CTRL and click on the RGB thumbnail in the Channels palette.  Alternatively you can press CTRL+ALT+2 together in PS CS5 or CTRL+ALT+~ in previous versions.  This selects your highlights and you’ll have a selection that looks like the one below.

Lighthouse Highlights Selected

The marching ants make this look like a discrete selection but it isn’t.  You’ll recall from the article on sharpening with luminance masks that these are very highly feathered selections.  With this selection made, simply hit the Delete key.  The overexposed highlight areas are deleted.  You’re left with a properly exposed sky and a visible foreground.  You can now make adjustments as you like to each layer separately to optimise the final image.  The image below is the blended result before any additional tweaking.

Lighthouse Blended

Try this simple but effective technique for yourself and let me know how you like it.  In a future article, we’ll explore luminance masks in a bit more detail for blending more complex images.



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