The Luminosity Blend Mode In Photoshop
On the previous page, we looked at Photoshop's Color blend mode which blends the color information (hue and saturation) of a layer with the layer or layers below it while ignoring the lightness values. As we saw, this makes the Color mode perfect for such things as colorizing black and white photos, and it's also great for more common tasks like changing someone's eye or hair color. The Color blend mode allows us to add or change colors in an image without affecting the brightness values.
Our fifth and final essential blend mode for photo editing in Photoshop is Luminosity. Like the Color mode, Luminosity is found in the Composite group of blend modes along with Hue and Saturation, and is actually the exact opposite of the Color mode. Whereas the Color mode blends the colors of a layer while ignoring lightness values, the Luminosity mode blends the lightness values while ignoring the color information!
In photo editing, changing the blend mode of a layer to Luminosity is often a final step. For example, a very common photo editing technique is to use either a Levels or Curves adjustment layer to improve overall contrast in an image, and in many cases, this works perfectly. The problem you can run into, though, is that Levels and Curves affect not only the lightness values in an image, they also affect color. By increasing image contrast, you're also increasing color saturation, especially in reds and blues, and sometimes you'll even see a shift in colors. Too much color saturation in a photo can wipe out important image details. By changing the Levels or Curves layer to the Luminosity blend mode, we easily avoid the problem by telling Photoshop to ignore the color information completely.
Real World Example of the Luminosity Blend Mode
Here we have a photo of a nice holiday table setting, full of reds, oranges and yellows:
I'm going to increase the contrast in this image using a Curves adjustment layer and a traditional "S" curve. I'll click on the New Adjustment Layer icon at the bottom of the Layers palette and choose Curves from the list of adjustment layers that appears:
Inside the Curves dialog box is a large 4x4 grid, with a diagonal line running through it from the bottom left to the top right. To change the shape of the diagonal line into a traditional "S" curve, I'll click on the line near the top right corner to add a point, then I'll nudge the point up a little by pressing the Up arrow key on my keyboard a few times. I'll then click on the line near the bottom left corner to add another point, then nudge the point down a little by pressing the Down arrow key on my keyboard a few times. This reshapes the line into something resembling a letter S (sort of, anyway), which is why it's known as an "S" curve:
I'll click OK to exit out of the dialog box. The "S" curve lightens the highlights in an image and darkens the shadows, which increases contrast, and as we can see in my image now, the contrast has been increased. Notice also, though, that the colors now appear more saturated as well, since the Curves adjustment layer affected not only the shadows and highlights but also the color saturation:
To have the Curves adjustment layer affect only the contrast and ignore the color information, all we need to do is change the blend mode of the adjustment layer from its default Normal to Luminosity:
Now that the blend mode is set to Luminosity, the Curves adjustment layer is no longer concerned with the color information in the image. The contrast is still increased, but the color saturation has returned to normal:
It may be a bit difficult to see the difference in the screenshots here, but try it on your own, switching between the Normal and Luminosity blend modes, and the difference will be easier to see, especially if your photo contains lots of reds and blues.
Another common use for the Luminosity blend mode is when it comes to sharpening images. Most people use Photoshop's classic Unsharp Mask filter to sharpen their photos, and there's certainly nothing wrong with using it. The only problem is that the Unsharp Mask filter sharpens both the lightness values and the color information, and this can lead to a more noticeable "halo" affect around people and objects in the image. We can use the Luminosity blend mode, along with Photoshop's Fade command, to limit the effects of the Unsharp Mask filter to just the luminosity values, ignoring the color.
Immediately after applying the Unsharp Mask filter, go up to the Edit menu at the top of the screen and choose Fade Unsharp Mask:
When the Fade dialog box appears, change the Mode option at the bottom (which is short for "blend mode") to Luminosity:
This effectively changes the blend mode of the Unsharp Mask filter you just applied to Luminosity, which means the filter is now safely ignoring the color information in the image and is sharpening only the lightness values. Add this extra step any time you apply the Unsharp Mask filter to give you better sharpening results!
And there we have it! Even though Photoshop includes up to 25 different layer blend modes depending on which version of Photoshop you're using, we've narrowed things down to just five blend modes you absolutely need to know. The Multiply blend mode darkens images, the Screen blend mode lightens images, the Overlay blend mode both darkens and lightens to improve contrast, the Color blend mode allows us to add or change colors in an image without affecting lightness values, and finally, the Luminosity blend mode allows us to make changes to the lightness values of an image without affecting color. Learning just these five essential blend modes can save you a tremendous amount of time and make editing, retouching and restoring photos in Photoshop a whole lot easier.