Luminosity and Color

By changing the mode of the layers to either Luminosity or Color we can restrict the corrections to where we want them to be. That allows us to make color corrections to an image without changing the density of a particular area, or modify brightness or contrast without influencing the color. The first image is a sunset with some modifications to each of the color channels in Levels to lighten or darken certain tones. This was done in Luminosity mode so the changes would not affect the colors in the image. Roll your mouse over the image to see the result with the Levels layer in the default Normal blend mode. Obviously, this would not be useful.

A density change in anything but neutral tones will always primarily modify the predominant color of that area compared to the others and introduce a color shift. This is usually ignored by most Photoshop users as it is minor in most cases. Minor or not, color shifts are usually a bad thing and fortunately are easy to avoid. Using individual channels while in Luminosity mode gives you the kind of control over the density of colors in the image that you had with filters in B&W film. Elements users can use the technique in Levels and Hue and Saturation adjustment layers to modify color densities. Photoshop users can add Curves, Black and White, and Selective Color adjustment layers to their tool chest. Other adjustment layers may also make minor modifications but are less effective.

In the second image we see the control that the Color blend mode affords us. In this case the correction that was needed was a curves adjustment to add magenta interest and saturation to the reflections of the sky in the surface of the pond. Existing color was there, but muted, so the desire was to enhance the color without affecting the brightness of the color values. This is easily done by changing the Curves layer blend mode to Color. This way the modifications applied to the curves to enhance the color are not applied to the image luminosity. Roll over the image to see the curves layer applied in the Normal blend mode. This could also be accomplished with Levels in Elements. It is important to note here that similar modifications can often be made using a Hue and Saturation adjustment layer, which is essentially giving you access to the same tools in a different manner. However, the modifications to the file are often different when applied through Curves or Levels in Color mode and knowing how to use both methods is valuable in being able to resolve specific issues.

The explanation for the ability to control luminosity separately from color is simple if you remember that Photoshop works in the L*a*b mode in the background. The L channel is luminosity, and the a and b channels are the green/magenta (tint) and yellow/blue (temperature) color channels respectively. While that is interesting to know, in almost all cases you will be working in RGB and not L*a*b, and knowing that you can make your editor ignore the color or luminosity with blend modes is the important part of the conversation.

To try the technique, if you are not familiar with layer modes, add an adjustment layer and click OK without making any corrections. In the upper left corner of the Layers dialog there is a drop down box that now says "Normal". Click on the arrow to the right and choose Luminosity to make changes to density or Color to adjust color without affecting the other. Both choices live at the bottom of a stack of blending choices, most of which you will not have much use for. Now select individual color channels and play around to see what you can do. Depending on the content of your image you will see some areas change more than others. The Luminosity mode is where you can be the most creative. Elements users do not have quite as much control as Levels restricts you to center point control, but even so, there are the ends and the output sliders as well to play with. Some changes will bring dramatic changes and others will be more subtle. This will be on an image by image basis as the amount of a particular color channel controlling various parts of the image will vary depending on how far from neutral the color is.

There are quite a few other blending modes as you can see on the drop down list. A few others are very useful for photography and will be explored in another article. In the meantime you can get used to using these two very useful blending modes to give you more control over how you edit your files.

This article and all images within it are ©2011 Bryson Leidich. All rights are reserved.
Opinions are those of the author. Correct appearance of sample images is dependent on your monitor.
www.brysonleidich.com