Luminosity Masks the Easy Way
March 1, 2016
Want to impress your photo buddies? Casually mention that you’ve started using luminosity masks in your Photoshop workflow, then enjoy their awed expressions. Surely that must be the secret to those stupendous images you keep coming up with!
Seriously, luminosity masks, which are just a way to select portions of an image based on brightness values, are rather popular in the Photoshop world right now. Indeed, they can be quite useful for a landscape photographer. A number of photographers offer downloadable Photoshop actions, or even complete new Photoshop panels, that let you automate the creation of a series of luminosity masks. There’s a simpler and more intuitive way to harness their power, however, one that doesn’t require downloading and installing anything, and that’s what I want to discuss in this blog post.
Luminosity masks are a way to make changes only to pixels that fall within a certain brightness range. In other words, they're useful when you want to select not a contiguous area of the image, or a particular color in the image, but instead want to select all pixels, regardless of their position within the image, whose brightness on a scale of 0 to 255 falls into a certain range. For example, you might want to add additional contrast to only the darkest regions of the image, the region where the average of the R, G, and B values of each pixel falls in the range 0 to 50. Once you’ve made the desired selection, you add an Adjustment layer. The layer mask that accompanies every Adjustment layer will automatically take on the characteristics of the active selection. Fully selected regions will be shown as white on the mask; unselected regions will be shown as black; and partially selected regions will be shown as some shade of gray. Your adjustments will be applied only to the region you’ve selected.
So how do you select only a particular part of the tonal scale? The trick is to use the Color Range selection tool. Most photographers use the Color Range tool to select specific colors in the image. For a number of years, Color Range also gave users the ability to select shadows, midtones, or highlights, but that aspect of the tool was limited because users had no control over what tones fell into those categories. Photoshop made all those decisions for you.
Photoshop CC added a welcome degree of flexibility to the selection process, which is now very useful indeed. In the top menu bar, under Select, choose Color Range. Once inside the Color Range dialog box (shown above), open the Select drop-down menu. Choose Highlights to work on the upper end of the tonal scale. You can specify the lower end of the range with the Range slider; you will be selecting all tones from that value up to blank white. Adjust the Fuzziness slider to control how much of the tones outside the range will be selected. In effect, you’re controlling how soft or hard the selection boundary will be. Values in the range 50 to 100 will generally give you the best results.
Similarly, you can choose Shadows in the Select drop-down menu and specify the upper end of the range of tones that will be selected. Everything from that tone down to pure black will be inside your selection.
Choose Midtones from the Select drop-down and you can specify both the lower and upper end of the range of tones that will be selected.
Best of all, you can see exactly what you’re doing by choosing one of the preview types in the Selection Preview drop-down. Choose Quick Mask to put a pink overlay over the non-selected parts of your image, as shown below. Selected portions will be shown in their natural colors. Choose Grayscale to see what the mask will look like after you add an adjustment layer with the selection active, remembering that white areas on the mask represent fully selected regions, while black areas are excluded from the selection and gray areas are partially selected.
I most often use this technique to add a bit of additional contrast to the shadows without adding more global contrast. Adding global contrast makes all the shadows darker and all the highlights brighter. That’s great if you want to add contrast to the mid-tones, but it can make the overall contrast too harsh and cause shadows to block up. After making my selection, I add a Curves adjustment layer and steepen the curve dramatically, as shown in the figure to the right. As I discussed in my previous post, a little bit of pure black is usually an asset in a landscape image, but too large a region of dark shadows frequently makes an image feel heavy or oppressive. Using this approach to luminosity masks allows me to retain a few completely black pixels while still maintaining good local contrast in the shadows.
Some caution is in order when using this approach. Occasionally this technique will generate peculiar local contrast somewhere in the image. If needed, just select the Brush tool and paint on the layer mask with black to remove the effect from the problem regions. It can also produce excessive color saturation. I frequently change the blend mode of the Curves Adjustment Layer to Luminosity so that I only affect brightness and contrast, not color. With a little care, this subtle method can make an image more lively while retaining a natural appearance.
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