In the July/August issue of DPP, we explored the fundamentals of blending channels, and we took a brief look at some classic strategies. This is a particularly powerful tool that you can use when you have big challenges, but it can create unplanned shifts. In this installment, we’ll address how to keep the effects under control.
Control The Mix With Blend Modes
As well as controlling the amount channels are blended, you can control the way they’re blended by using blending modes. Blend modes determine how new values are mixed with old values. There are dozens of blend modes to choose from.
As color adjustment is achieved by altering the luminance (light and dark) values of select channels (Channels create but don’t contain color or saturation), when it comes to blending channels, you can limit the number of blend modes you use to those that affect tone. Five are particularly useful—Lighten, Screen, Darken, Multiply and Luminosity.
Lighten displays the lightest values of both This Layer and the Underlying Layer; its neutral color is black (you can’t lighten with black).
Screen multiplies the inverse values of the pixels’ lightness or darkness. It’s like registering the same image in the same location from two projectors. Think of it as industrial-strength lightening. Its neutral color is black (you can’t lighten with black). Screen can do wonders for opening up deep shadows. It has a tendency to blow out highlights. Use a contrast mask to remove the effect from highlights.
Darken displays the darkest values of both This Layer and the Under-lying Layer; its neutral color is white (you can’t darken with white).
Multiply multiplies the values of the pixels on both layers and then divides by 255. It’s like registering two identical transparencies on a light table. Think of it as industrial-strength darkening. Its neutral color is white (you can’t darken with white). Multiply can do wonders for reclaiming subtle highlight detail. It has a tendency to block up shadows. Use a contrast mask to remove the effect from shadows.
Luminosity combines the luminance values of This Layer with the hue and saturation of the Underlying Layer; it has no neutral color.
Enhance The Blend
You can enhance a channel before (or if you use channels as layers after) blending it with another. Use any adjustment method that makes the data better to blend with. As you’re blending with black-and-white images, Curves is usually all you need, for it offers the most precise control of tone. For instance, you might increase the contrast of an image before using it to blend with. If you’re using the channels-as-layers method, all you have to do is group a Curves adjustment layer to the new layer being used to blend with. The contrast of the overlying layer can then be fine-tuned as the blend with the underlying layer is occurring. This way you don’t have to guess how much contrast needs to be added before blending, instead you see how much contrast to add while the blend is occurring.
Constraining The Effect
While blending channels may solve problems that other adjustment methods can’t, they may also produce new problems.
Multiply multiplies the value of the pixels on both layers and then divides by 255. It’s like registering two identical transparencies on a light table.
In a great many cases, if the tonal distribution of a single channel is substantially altered using another channel, color may shift in an unintended manner. If this happens, simply make an additional adjustment to eliminate any side effects. There are times when the color shifts you encounter will be non-uniform (more in some areas than others), which may lead you to making more complex corrections than you had anticipated.
If the problem solved with channel blending and the resulting side effects lie in different areas of the image, consider masking away the side effects rather than correcting them. There are several ways of masking the side effects of channel blending from selected areas. One, simply brush them away by painting with a black brush on a layer mask. Two, use a contrast mask to hold back the effect from highlights or shadows. Three, use the Blend If function in Layer Styles; by sliding the black arrow to the right or the white arrow to the left, you drop out the effects from values below or above them—by holding the Option key (Command on PC), you can split the sliders to fade the effect more smoothly.
If you think you’re not used to blending channels, think again. Every time you turn a color image into a black-and-white image, you blend channels. In a grayscale conversion, three channels are blended to create a single channel, while when using either Hue/Saturation or Channel Mixer, three channels are blended to equal RGB values. But, when it comes to color adjustment, blending channels is used infrequently, perhaps because it’s so little known. Blending channels is a sophisticated adjustment method. In a majority of cases, you don’t need a method that’s this complex. Blending channels is best used in exceptional situations for enhancing originals with substantial problems. If you find that you use this technique frequently, you’re probably not addressing the real problem—the quality of your originals. Nevertheless, when you run into files with severe problems, blending channels will often save the day.
1. Start with four copies of one image.
2. Convert three copies of one image into three different color spaces—LAB, RGB and CMYK.
3. Split Channels command to preview all the channels simultaneously.
4. Identify the problem channel.
5. Identify the channel that contains the best replacement information.
6. Hold the Shift key and drag the background layer of that channel (now a separate grayscale document) into the window of the color original. A new pin-registered layer will appear in the Layers palette.
7. Double-click on the new layer’s icon activating the Layer Style dialog box.
8. Under Advanced Blending, check the channel to be affected.
9. Under Blending Options, set Blend Mode and Opacity.
10. Under Blend If, use the sliders to drop out the effect in shadows or highlights, if necessary.
11. Mask, if n
12. For added effect, correct the tone of the new layer by grouping a Curves adjustment layer to it.
13. Repeat with the other channels as necessary.
John Paul Caponigro, author of Adobe Photoshop Master Class and the DVD series R/Evolution, is an internationally renowned fine artist, an authority on digital printing, and a respected lecturer and workshop leader. Get access to a wealth of online resources with his free enews Insights at www.johnpaulcaponigro.com.