Real World Adobe Photoshop CS2: Industrial-strength Production Techniques

Photoshop's Duotones

There are two ways to create multitone images in Photoshop: Switch from Grayscale to Duotone under the Mode submenu, or create them in CMYK mode. We're focusing our discussion on the Duotone mode, and will cover CMYK mode later in this chapter.

In most color modes, each pixel's color is described with multiple channelsCMYK color is made of four channels, RGB is made of three. Multitones are different. In a multitone image, Photoshop saves a single grayscale image along with two or more curvesone for each color plate. These curves are just like those in the Curves and Transfer Function dialog boxes (see Figure 10-6). Note that you can only create a duotone from an 8-bit-per-channel grayscale image. (If you've got a color image, see "The Color of Grayscale" in Chapter 12, Essential Image Techniques.)

Because you're typically replacing a single gray level with two or more tints of ink, you almost always need to adjust the amounts of ink used by each channel. Otherwise, the image appears too dark and muddy (see Figure 10-7). For instance, if you replace a 50-percent black pixel with 50-percent black and 50-percent purple, it appears much darker. Instead, replacing that 50-percent black with something like 30-percent black and 25-percent purple maintains the tone of that pixel. On the other hand, if the second color were much lighter, like yellow, you'd need much more ink to maintain the tone. You might, for example, use 35-percent black and 55-percent yellow.

The duotone curves give you the ability to make these sorts of tonal adjustments quickly and with a minimum of image degradation because applying a duotone curve never affects the underlying grayscale image data. You can make 40 changes to the duotone colors or the curves and never lose the underlying image quality.

Note that we say the "underlying image quality" won't suffer. We're not saying that you can go hog-wild with the curves, and your final image will always look good. Far from it. In fact, duotones, tritones, and quadtones are often very sensitive and can quickly succumb to "lookus badus maximus." But the image data saved on disk is unchanged by adjusting these curves; they're like filters that are applied to the image data, but only when you view it on screen or print it out.

Tip: Use Stephen's Curves

Unless you really know what you're doing, just use the duotone curve sets built by photographer Stephen Johnson that ship with Photoshop. We almost never create a multitone image from scratch. Instead, we click the Load button in the Duotone dialog box, navigate to the Duotones folder (inside the Presets folder in the Adobe Photoshop folder), and pick one from there. Then we make small tweaks to the curves, depending on the image.

Most of the curves come in sets of four.

• The first and second colorize the image (the first does so more than the second).

• The third curve of the set affects the midtones and three-quarter tones primarily, and does very little to the highlights. The effect is to warm or cool the image significantly without colorizing it much.

• The final duotone curve makes the image slightly warmer or cooler (still mostly neutral), primarily affecting the three-quarter tones.

If we're using a Pantone color that's not included in the canned presets, we usually pick a canned set for a color that has similar brightness to the one we're using, and replace the color with ours. Then, depending on the two colors' tones, we adjust the curves accordingly.

Tip: Checking Each Duotone Channel

The biggest hassle with images in Duotone mode is that you can't see each channel by itself. If you make a tonal adjustment to an RGB or CMYK image, you can always go and see what that did to each channel. With a duotone, however, you're in the dark. Here's one way out: Convert the image to Multichannel mode.

While you're in Multichannel mode, you can view each channel of the multitone image. The first channel corresponds to the first ink, the second to the second ink, and so on. When you're done playing voyeur, select Undo (Command-Z), and you're back where you started.